AD-A023 513
ENGINEERING DESIGN HANDBOOK: RECOILLESS RIFLE
WEAPON SYSTEMS
Army Materiel Command
Alexandria, Virginia
15 January 1976
T
DISTRIBUTED BY:
National Technical Information Service
U. S. DEPARTMENT OF COMMERCE
11918 - 1 —'
AMC PAMPHLET
AMCP 706-238
)
40 4oQ 3 s/s
ENGINEERING DESIGN
HANDBOOK
RECOILLESS
RIFLE
WEAPON SYSTEMS
HEADQUARTERS, US ARMY MATERIEL COMMAND
JANUARY 1976
REPRODUCED BY
NATIONAL TECHNICAL
INFORMATION SERVICE
U. $. DEPARTMENT Of COMMERCE
SPRINBf IELD, VA. SIR
AHi 7 127S
AMCP 706-238
DEPARTMENT OF THE ARMY
HEADQUARTERS UNITED STATES ARMY MATERIEL COMMAND
5001 Eisanhowar Ava, Alexandria, VA 22333
AMC PAMPHLET
No. 706-238
15 January 1976
ENGINEERING DESIGN HANDBOOK
RECOILLESS RIFLE WEAPON SYSTEMS
TABLE OF CONTENTS
Paragraph
Page
UST OF ILLUSTRATIONS . xix
LIST OF TABLES . xxvii
PREFACE . xxix
PART ONE INTRODUCTION
CHAPTER 1 BACKGROUND INFORMATION
SECTION I SCOPE. 1-1
SECTION II HISTORY
1-1 General . 1-3
1 -2 History to End of World War II . 1-3
1-2.1 Development Prior to 1943 . 1-3
1-2.2 Development of 57 mm Rifle, M18 . 1-3
1-2.3 Development of 75 mm Rifle, T21 (M20). 1-9
1 -2.4 Development of 105 mm Rifle to End of World
WarH . l-l 1
1-3 History Post-World War II . 1-11
1-3.1 Development of 105 mm Rifle, T19 (M27) . 1-11
1 -3.2 Development of 106 mm BAT Weapon System .... 1-12
1-3.2.1 Development at Frankford Arsenal. 1-12
1-3.2.2 Development at Firestone . 1-14
l -3.2.3 Development of 106 mm Rifle, M40 . 1-14
1-3.2.4 Development at Frigidaire . 1-15
1-3.2.5 Spotting Rifle Development . 1-16
1-4 Other Recoilless Weapons of Caliber 105 mm or
Smaller . 1-17
1-4.1 37 mm Rifle, T62. 1-17
1-4.2 57 mm Rifle, 166. 1-17
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1-4.3 2.7S-in Rifle, T190 . 1-18
1-4.4 90 mm Rifle and Ammunition . 1-19
1 -4.5 Development of Repeating Rifles 105 mm, T189 and
T237 . 1-22
1-4.6 Development of 105 mm Rifle, T136 1-26
1 -4.7 Development of Weapon System T165 and T166, Self-
propelled (ONTOS) Using 106 mm, T170 Recoilless
Ritie . 1-26
1 -5 Other Large Caliber Weapons (Larger than 105 mm) .. 1 -30
1-5.1 Development of 120 mm HAW. 1-30
1-5.2 DAVY CROCKETT 120 mm, XM63 (XM28) and 155
mm, XM64 (XM29) 1-32
1-5.3 Development of 8-in. Cannon (EIK). 1-35
1-5.4 Development of Self-ejecting Breech. 1-37
1-6 Research Programs . 1-37
1-6.1 Introduction. 1-37
1-6.2 Midwest Research Institute . 1-38
1-6.2.1 Gun Temperature . 1-38
1-6.2.2 Sheet Propellant Studies. 1-38
l-6.2.3 Gun Dynamics . 1-39
1 -6.2.4 Ignition Studies . 1 -39
1-6.2.5 Flash Characteristics . 1-39
1-6.3 Armour Research Foundation . 1-40
1-6.3.1 Interior Ballistic Theory . 1-40
1-6.3.2 Propellants . 1-41
1-6.3.3 Expendable Cartrioge Case . 1-41
1-6.3.4 Nozzle Studies . 1-42
1-6.3.5 Stress Analysis . 1-43
1-6.4 Firestone Tire and Rubber Company . 1-44
1-6.4.1 Aerodynamics. 1-44
1-6.4.2 Fuze Studies . 1-44
1-6.5 Universal Winding Company. 1-44
1-6.6 A. D. Little, Inc. 1-45
1-6.7 Harvey Aluminum (Harvey Machine Co.) . 1-45
1-6.8 CARDE. 1-46
1- 6.9 Franklin Institute . 1-46
References . 1-47
CHAPTER 2 SYSTEM DESIGN AND INTEGRATION
2- 0 List of Symbols . 2-1
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SECTION I INTRODUCTION
2-1 Scope . 2-3
2-2 Definition of Terms . 2-3
2-3 General Principles of Operation . 2-4
SECTION II SYSTEM REQUIREMENTS
2-4 General . 2 9
2-5 Required Muzzle Energy. 2-9
2-5.1 Kill Probability. 2-9
2-5.2 Hit Probability . 2-9
2-5.3 Vulnerable Area . 2-11
2-6 Weapon System Weight . 2-11
SECTION Ill DETERMINATION OF
BALLISTIC PARAMETERS
2-7 Determine Throat Area . 2-13
2-8 Determine Gun and Propellant Requirements. 2 - 13
2-9 Verify Calculations With Test Weapon . 2-15
2- 10 Complete Design of Gun, Round, and Ancillary
Equipment . 2-16
SECTION IV NUMERICAL EXAMPLE. 2-17
References . 2-21
PART TWO THEORETICAL ANALYSIS
CHAPTER 3 TERMINAL BALLISTICS
3- 0 List of Symbols . 3-1
SECTION I INTRODUCTION
3-1 Scope . 3-3
3-2 Background. 3-3
3-3 Typical Recoilless Warheads .. 3-3
SECTION II HEAT WARHEAD
3-4 Qualitative Description . 3-7
3-5 Factors Affecting Performance . 3-7
3-5.1 Introduction. 3-7
3-5.2 Projectile Spin . 3-8
3-5.3 Physical Properties of Liner . 3-9
iii
TABLE OF CONTENTS (Cont'd)
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3-5.4 Standoff . 3-10
3-5.5 Cone Angie . 3-11
3-5-6 Liner Wall Thickness. 3-12
3-5.7 Liner Shape . 3-12
3-5.8 Alignment of Cone and Charge. 3-12
3-5.9 Confinement. 3-13
SECTION III HE WARHEAD
3-6 Qualitative Description . 3-15
3-7 Determination of Fragmentation Characteristics. 3-15
3-7.1 Fragment Size Distribution . 3-15
3-7.2 Initial Fragment Speed. 3-16
3-7.3 Fragment Slow Down . 3-18
3-7.4 Fragmentation Patterns. 3-18
3-7.5 Controlled Fragmentation. 3-20
3-7.5.1 Preformed Fragment. 3-20
3-7.5.2 Notched or Grooved Rings. 3-21
3-7.5.3 Notched or Grooved Wire . 3-21
3-7.5.4 Notched Casings. 3-22
3—7.5.5 Multiple Walls. 3-22
3-7.S.6 Metallurgical^ Modified Material . 3-22
SECTION IV OTHER TYPES OF WARHEADS
3-8 HEP Warhead . 3-23
3-8.1 Introduction. 3-23
3-8.2 Advantages and Disadvantages . 3-23
3-8.3 Theory of Performance. 3-24
3-8.4 General Conclusions . 3-24
3- 9 Other Types of Warheads . 3-25
References . 3-25
CHAPTER 4 EXTERIOR BALLISTICS
4— 0 List of Symbols . 4-1
SECTION 1 INTRODUCTION
4-1 Scope . 4-5
4-2 Weapon System Interaction . 4-5
4-3 Qualitative Description . 4-5
TABLE OP CONTENTS (Cont'd)
Paragraph Page
SECTION II AERODYNAMIC FORCES
AND MOMENTS
4-4 General . 4-7
4-5 Aerodynamic Forces . 4-7
4-5.1 Normal, Lift, and Drag Forces . 4-7
4-5.2 Magnus Force . 4-7
4-6 Aerodynamic Moments . 4-7
4-6.1 Static Moment . . 4-7
4-6.2 Damping Moment. 4-8
4-6.3 Magnus Moment . 4-8
4-6.4 Roll Damping Moment . 4-8
4-7 Force and Moment Coefficients . 4-9
4-7.1 Aerodynamic Force Coefficients . 4-9
4-7.2 Moment Coefficients and Moments . 4-10
4-8 Determination of Aerodynamic Coefficients . 4-11
SECTION III PROJECTILE STABILITY
4-9 Introduction . 4-13
4-10 Basic Stability Considerations. 4-13
4-11 Spin Stabilization . 4-13
4-11.1 Gyroscopic Stability. 4-13
4-11.2 Yaw of Repose . 4-14
4-11.3 Dynamic Stability. 4—15
4-11.4 Aerodynamic Jump of Spin-stabilized Projectiles .... 4-15
4— i2 Fin Stabilization.. 4-17
4-12.1 Introduction. 4-17
4-12.2 Fin Types. 4-17
4-12.3 Dynamic Stability. 4-18
4-12.4 Aerodynamic Jump of Fin-stabilized Projectiles .... 4—18
4-12.5 Magnus Stability . 4-19
4-12.6 Resonance Instability . 4-19
SECTION IV AERODYNAMIC DRAG
4-13 General . 4-21
4-14 Subsonic Velocities. 4-22
4-15 Transonic . 4-23
4-16 Supersonic . 4—23
4-17 Typical Values of Drag . 4-23
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SECTION V PARTICLE TRAJECTORY
CALCULATIONS
4-18 Trajectory Problem. 4-27
4-19 Trajectory Equations . 4-27
4-20 Solutions of the Equations . 4-27
4-20.1 Semicmpirical Equations for Flat Trsyectories. 4-28
4-20.2 Digital Computer Solutions . 4-29
4-20.3 Other Methods . 4-30
4-20.3.1 Numerical Integration. 4-30
4- 20.3.2 Siacci Tables . 4-30
References . 4-34
CHAPTER 5 INTERIOR BALLISTICS
5- 0 List of Symbols . 5- 5
SECT ION I INTRODUCTION
5-1 Scope . 5-7
5-2 Qualitative Description of the Interior Ballistic
Problem . 5-7
5-3 Use of Existing References on Interior Ballistic Theory . 5-9
5-4 Design Data for Several Recoilless Rifles and Ammuni¬
tion . 5-9
SECTION II EMPIRICAL AND GRAPHICAL
METHODS FOR QUICK
APPROXIMATIONS
5-5 Solutions Based on Efficiency Considerations. 5-11
5-5.1 Introduction. 5-11
5-5.2 Thermodynamic Efficiency . 5-11
5-5.3 Piezometric Efficiency . 5-12
5 -5.4 Efficiency Tables and Graphs . 5—12
5-5.5 Numerical Example . 5-13
5-6 Tabulated Design Data . 5-14
5-6.1 Method. 5-M
5-6.2 Example . 5-18
5-7 Graphical Solutions. 5-19
5-7.1 Introduction. 5-19
5-7.2 Procedure for Using Graphs . 5-25
5-7.3 Numerical Example . 5-29
5-8 Similitude Relations . 5-29
5-8.1 Introduction. 5—29
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S-8.2 Characteristic Similitude Relations . 5—31
5-9 Effect of Ballistic Variations . 5-31
5-9.1 Introduction. 5-31
5-9.2 Effect of Quickness Factor B/W 0 . 5-32
5-9.3 Effect of Impetus F . 5-32
5-9.4 Effect of Propellant Regressiveness W/L . 5-33
5-9.5 Effect of Flow Factor T . 5-33
SECTION III BASIC INTERIOR BALLISTIC
EQUATIONS
5-10 Equations for Projectile Acceleration . 5-35
5-11 Equation of State for Propellant Gas . 5-35
5-12 Equation for Rate of Propellant Burning . 5-37
5-13 Equation for Dischatge of Propellant Gas Through
Nozzle . 5-41
5-14 Equation for Accumulation of Gas in Gun . 5-41
5-15 Energy Equation. 5-41
5-16 Summary of Equations . 5-42
SECTION IV DISCUSSION OF SOLUTION
TO EQUATIONS . 5 45
SECTION V SIMPLE SOLUTION BASED ON
CONSTANT AVERAGE TEMPERATURE
5-17 Introduction . 5-47
5-18 Method . 5-47
5-19 Example . 5-48
SECTION VI ANALYTIC EQUATIONS FOR
OPTMIZING CERTAIN GUN PARAMETERS
5-20 The Lightest Gun for a Specified Muzzle Energy .... 5-51
5-21 The Shortest Gun for a Specified Muzzle Velocity .... 5-55
5-22 Numerical Example. 5-55
SECTION VII INTERIOR BALLISTIC SOLUTION
USING DIGITAL COMPUTER. 5-57
SECTION VIII SOLUTION FOR AFTER
“ALL-BURNT” CONDITION
5-23 Introduction . 5-59
5-24 Modification of Equations for “All-Burnt” Condition . . 5-59
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S-2S Solution of Equations for “All-Burnt” Condition .... S-S9
5-26 Example. 5-60
SECTION IX HEAT TRANSFER
5-27 Introduction . 5-61
5—28 Basic Equations . 5-61
5-29 Solution of the Equations . 5-62
5—30 Temperature Distribution Data . 5—64
5-30.1 Theoretical Calculation. 5-64
5—30.1.1 Single Shot Analysis . 5-64
5—30.1.2 Determination of Temperature as a Function of
Round Number and Rate of Fire .. 5—64
5—30.2 Experimental Phase . 5—78
SECTION X SPECIAL TOPICS
5-31 Loss of Unbumt Propellant. 5-81
5-32 Pressure Gradient in Gun . 5-83
5-33 Form Factor for Propellant Burning . 5-83
5-34 Muzzle Flash . 5-85
5-34.1 Basic Theory. 5-85
5-34.2 Flash Suppression. 5-85
5-35 Calculation of “Bare” Gun Weight . 5-86
5— 36 List of Numerical Constants Used in Interior Ballistic
Calculations . 5-87
References . 5-87
Bibliography . 5-88
CHAPTER 6 CANCELLATION OF RECOIL
6- 0 List of Symbols . 6-1
SECTION I INTRODUCTION
6-1 Conservation of Momentum . 6-5
6-2 The Supersonic Nozzle . 6-5
6-3 Effect on Interior Ballistics. 6-6
SECTION II THEORY OF THE DE LAVAL
(CONVERGENT-DIVERGENT) NOZZLE
6-4 Assumption . 6-9
6-5 Definitions . 6-9
MKT'MW
TABLE OF CONTENTS (Corn'd)
Anagraph Page
6-6 Buie Equations . 6-30
6-6.1 Rate of Flow. 6-10
6-6.2 Matt Flow. 6—12
6-6.3 Thrust Generated by Nozzle. 6-13
6-7 Design Considerations. 6-IS
SECTION III THEORY OF RECOIL
CANCELLATION
6-8 Definition of Momentum Ratio Parameter . 6-21
6-9 Equation for Momentum Ratio as a Function of Gun
and Nozzle Parameters . 6-22
6-10 Equations for Ratio of Chamber Pressure to Ideal
Reservoir Pressure. 6-22
6-11 Graphical Solution of the Equations . 6-23
6-12 Nozzle Performance Factors . 6-24
6-12.1 Variation of Nozzle Thrust With Nozzle Expansion
Angle . 6-24
6-12.2 Variation of Nozzle Thrust With Expansion Ratio ... 6-25
6-12.3 Effect of Nozzle Approach Area and Chamber Con¬
figuration on Rifle Performance . 6-27
SECTION IV NOZZLE EROSION
6-13 General Discussion . 6-31
6-14 Theory . 6-31
6-IS Erosion Resistance of Various Metals . 6-32
6-16 Similitude Relationships. 6-36
6-17 Other Factors That Affect Erosion Rate. 6-37
SECTION V BORE-SIZE NOZZLE . 6- 39
SECTION VI RECOIL COMPENSATORS . 6-41
SECTION VII BLAST EFFECTS
6-18 Introduction . 6-43
6-19 Various Damage Mechanisms . 6-43
6-20 Blast and Flash Patterns . 6-44
6-21 Experimental Data . 6-49
6-21.1 Pressure Contours. 6-49
6-21.2 Danger Areas. 6-51
6-21.3 Ducting. 6-51
References . o-53
Bibliography . 6-55
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CHAPTER 7 SYSTEM EFFECTIVENESS
7-0 List of Symbols . 7-1
SECTION I INTRODUCTION . 7-3
SECTION II HIT PROBABILITY
7-1 General . 7-5
7-2 Sources of Error. 7-5
7-3 Calculation of Hit Probability. '/-6
7-3.1 General . 7-6
7-3.2 Errors Associated With Type of Fire Control System . 7-7
7-3.3 Lateral and Vertical Single Shot Hit Probabilities .... 7-8
7-4 Use of Spotting Round . 7-9
7-4.1 General . 7-9
7-4.2 Magnitude of Mismatch. 7-10
7-5 Probability of Hit With Recoiiless Rifles. 7-11
7-5.1 Comparison of Simple Sight and Spotting Round .... 7-11
7 -5.2 Probability of Hit for Standard Weapons . 7-11
7-5.3 Probability of Hit as a Function of Various Conditions 7-18
7 -5.4 Probability of Hit as a Function of Muzzle Velocity .. 7-18
SECTION III KILL PROBABILITY
7-6 Introduction . 7-23
7-7 Hard Target. 7 23
7-7.1 Introduction . 7-23
7-7.2 Types of Kill. 7-23
7-7.3 Vulnerable Area . 7-23
7 7.4 Calculation of Kill Probability . 7-24
7- 7.5 Typical Values of Kill Probability. 7-25
7-8 Area Target. 7-25
7-8.1 Introduction . 7-25
7- 8.2 Lethal Area . 7-25
References . 7—27
Bibliography . 7-27
CHAPTER 8 MEASUREMENT TECHNIQUES
8- 0 List of Symbols . 8-1
SECTION I INTRODUCTION . 8-3
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iLvJlION II MEASUREMENT OF VELOCITY
8-1 General . 8-5
8- 2 Detecting Devices . 8-6
8-2.1 Breakwire System. 8-7
8-2.2 Make System. S-8
8-2.3 Solenoid Coil Detectors . 8-10
8-2.4 Sky Screen . 8—11
8-2.5 Radar Velocity Measurements . 8-11
8-2.6 Photographic Methods . 8-13
SECTION 111 PRESSURE MEASUREMENTS
8-3 General . 8— 17
8—4 Copper Crusher Gage . 8-17
8-5 Piezoelectric Gage . 8-18
8-6 Strain Gages . 8-18
SECTION IV OTHER MEASUREMENT
TECHNIQUES
8-7 Strain Measurements . 8—21
8-7.1 General. 8-21
8 -7.2 The Gage . 8-21
8-7.3 Other Uses of Strain Gages . 8-21
8—8 Acceleration Measurement . 8-22
8-8.1 General. 8-22
8-8.2 Accelerometers. 8-22
8—9 Recoil Measurements . 8-23
8—9.1 General. 8—23
8—9.2 Measurement of Recoil Impulse . 8-24
8-9.3 Measurement of Recoil Forces . 8-24
8-10 Measurement of Temperature. 8-24
8—10.1 General. 8-24
8-10.2 Techniques . 8-24
8-11 Projectile Motion . 8-24
8-11.1 Yaw . 8-24
8-11.2 Spin . 8-25
8-12 Blast . 8-26
8-12.1 General. 8-26
8—12.2 Blast Gages . 8-26
8—13 Recording Equipment. 8-28
8—13.1 Oscilloscope . 8-28
8-13.2 Magnetic Tape . 8-28
SECTION V GENERAL CONSIDERATIONS
References . 8-31
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PART THREE DESIGN
CHAPTER 9 BASIC DESIGN CONSIDERATIONS
SECTION I INTRODUCTION TO DESIGN
CONSIDERATIONS
9— 1 Advantages of Recoilless Rifles . 9—1
9-2 Importance of System Design Approach. 9-1
9-3 Description of Various Weapon Configurations . 9-2
9-3.1 Basic Principle . 9-2
9-3.2 The Davis Gun . 9-2
9-3.3 Russian and German Designs. 9-3
9-3.4 The Burney Gun . 9-5
9-3.S The Hybrid Weapon . 9- "
9-3.6 Side-loading Configuration . 9-5
9-3.7 Configuration With Perforated Cartridge Case . 9-10
9-3.8 Special Configurations . 9-15
9-4 Disadvantages. 9-15
SECTION II HUMAN ENGINEERING
9-5 Introduction . 9-37
9-6 Primary Factors . 9-37
9-6.1 The Man Using the Weapon . 9-37
9-6.2 Field Servicing . 9-39
9—6.3 Manufacturing Personnel . 9-39
9-7 Human Factors Engineering Evaluation. 9-40
9—8 Areas of Application . 9—40
9-9 Specific Responsibilities . 9-40
SECTION III RELIABILITY
9-10 Basic Principles . 9-43
9-11 Materials. 9-44
9-12 Environmental Deterioration . 9-45
SECTION IV MAINTAINABILITY
9-1V Basic Principles . 9-49
9-14 Accessibility . 9-49
9-15 Standardization . 9-50
References . 9-50
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CHAPTER 10 RIFLE AND RIFLE
COMPONENTS
10-0 list of Symbols . 10-1
SECTION I OVERALL DESIGN
CONSIDERATIONS
10-1 General . 10--3
10—2 Hammer Blow. 10-3
10—3 Firing Pin . 10-3
10-4 Primer. . 10-4
10—5 Booster . 10-4
10—6 Propellant . 10-4
10-7 Cartridge Case. 10-5
10—8 Projectile. 10-5
10—9 Breech-Cartridge Relation . 10-6
10-10 Chamber-Cartridge Relation . 10-6
10—11 Tube-Cartridge Relation . 10-6
10—12 Chamber. 10-6
10—13 Nozzles . 10-7
10-14 Tube .:. 10-7
10-15 Summary . 10-8
SECTION II NOZZLE
10-16 General . 10-9
10-17 Nozzle Erosion . 10-9
10-18 Various Types of Nozzles . 10—10
10-18.1 Central Nozzle . 10-10
10-18.2 Central Nozzle With Bar . 10-12
10-18.3 Central Expanding Nozzle. 10-13
10-18.4 Multiple Nozzle and Front Orifice. 10-13
10-18.5 Annular Nozzle. 10-14
10—18.6 Interrupted Annular Nozzle . 10-16
10—18.7 Kidney-shaped Nozzle . 10-16
SECTION III BREECH
10-19 General . 10-19
10-20 Characteristics. 10-19
10-21 Sealing Propellant Gases . 10-20
10-22 Breech Types . 10-20
10—23 Breech Act 1 lator . 10-22
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SECTION IV CHAMBER
10-24 General . 10-23
10-25 Significance of Chamber Volume . 10-23
10—26 Ejection of Propellant. 10—23
SECTION V TUBE
10-27 General . 10-25
10-28 Design Considerations. 10-25
10-29 Other Subjects To Be Considered in Design . 10-26
SECTION VI FIRING MECHANISM
10-30 Design Characteristics. 10—29
10-31 Examples . 10-29
10— 32 Safety Devices. 10—31
References . 10-37
Bibliography . 10-38
CHAPTER 11 AMMUNITION
11 -0 List of Symbols . 11-1
SECTION I GENERAL
11- 1 Introduction . 11-3
11 -2 Overall Design Considerations. 11-3
11 -3 List of Existing Cartridges With Characteristics . 11-5
SECTION II PROJECTILE
11 -4 Introduction . 11-7
I i —5 Projectile Typ'« . 11-7
II -6 Design Considerations. 11-10
11 -6.1 Envelope . 11-10
11-6.2 Required Informal n . 11 — 10
11 -6.3 Method of Stabilization . 11-11
11 -7 Metal Parts Security-Structural Integrity Within the
Ballistic Environment . 11-11
11-7.1 General. 11-11
11-7.2 Stress Analysis . 11-12
I 8 Aerodynamic Design . 11-13
II -9 Other Design Considerations . .... 11 — 13
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11-10 Warhead Design . 11-15
11-11 RotatingBand. il-15
11-12 Obturators . 11-15
11-13 Strain Compensation . 11-16
11-14 Shot Start . 11-16
11-15 Spigots . 11-16
SECTION III CARTRIDGE CASE
11-16 Introduction . 11-17
11-17 The Perforated Cartridge Case. 11-17
11-17.1 General. 11-17
11-17.2 Effect on Interior Ballistics . 11-19
11 -17.3 Effect of Perforation Hole Diameter . 11-19
11 -17.4 Pressure Differential Across Cartridge Case. 11 -23
11-17.5 Stress Analysis . 11-24
11 -17.6 Liners for the Perforated Cartridge Case . 11—28
11 -17.7 Materials for Liners . 11 -29
11 -17.8 Applications of Liners . 11 -30
11-18 The Frangible Cartridge Case . 11-31
11-18.1 General. 11-31
11-18.2 Requirements. 11-31
11 -18.3 Materials for Frangible Case . 11-32
11-18.4 The DAVY CPOCKETT Cartridge Case . 11-32
11-19 The Unperforated Cartridge Case . 11-33
SECTION IV IGNITER
11-20 Introduction . 11-35
11-20.1 Scope . 11-35
11-20.2 Background . 11-35
11-21 Igniter Configuration . 11 -36
11-21.1 General. 11-36
11 -21.2 Secondary Igniter Charge . 11 -36
11-21.3 Main Igniter Charge . 11 -36
11 -21.4 Primer Adapter and Ignition Tube . 11 -36
11-21.5 Primer . 11-39
11-21.5.1 Small Arms Percussion Frimers . 11-39
11-21.5.2 Artillery-type Printers. 11-40
11-22 Basic Design Information . 11 -40
11-23 Development Procedure . 11-41
11-23.1 General. 11-41
11 -23.2 Determination of Hole Size and Pattern . 11-41
11 -23.3 Sample Calculations . 11 -43
11 -23.4 Selection of Hole p attem . 11 -44
xv
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11 —23.5 Preliminary Ballistic Testing. 11 -44
11-23.6 Final Engineering Testing. 11-45
SECTION V THE FUZE
11-24 General . 11-49
11—25 Type of Fuzing ... 11-49
11 -26 Safe-Arm Separation . . 11 -49
SECTION VI PROPELLANT
11-27 Introduction . 11-51
11-28 History . 11-52
11-29 Basic Characteristics . 11-52
11-29.1 Propellant Compositions . 11-52
11-29.2 Impetus. 11-53
11—29.3 Flame Temperature . 11-53
11-29.4 Web Thickness . 11-53
11 -29.5 Burning Rate. 11 -53
11 -29.6 Propellant Shape . 11 -54
11 —30 Chemical and Physical Characteristics. 11 -54
11- 31 Progressive and Regressive Burning. 11-56
References . 11-56
Bibliography . 11-57
CHAPTER 12 MOUNTS
SECTION I INTRODUCTION
12- 1 General . 12-1
12-2 Specific Examples . 12-3
12-2.1 M79 Mount . 12-3
12-2.2 T173 Mount . 12-4
12-2.3 XM124 Mount . 12-5
12-2.4 T234 Mount . 12-5
SECTION II ACCESSORY MOUNTING
EQUIPMENT
12-3 General . 12—9
12-4 Mounting Methods . 12-9
12 -4.1 Moderately Stressed Weapons. i 2—9
12-4.2 Highly Stressed Weapons . 12-10
12-5 Mounting Requirements. . 12-10
AMCP 706>23B
TABLE OF CONTENTS (Cont'd)
Barograph Page
12-5.1 Ground and Vehicular Mounts . 12-10
12-5.2 Telescope Mount . 12-13
12- 5.3 Spotting Rifle Mount . 12-13
References . 12-17
CHAPTER 13 FIRE CONTROL
13- 1 General . 13-1
13-2 Typical Designs . 13-2
13-2.1 106 mm Rifle, M40 With Cal .50 Spotting Rifle, M8C . 13-2
13-2.2 120 mm Rifle, XM105 With Spotting Rifle, XM90E1 . 13 -3
13-3 Types of Spotter-Tracer Rounds. 13—3
13-4 Evaluation of Target Display . 13-4
13-5 Compositions . 13-4
13-6 Ignition . 13-5
References . 13-9
Bibliography . 13-9
Index . 1-1
xvii
AMCP 706*238
LIST OF ILLUSTRATIONS
Figure Title Page
1-1 Rifle, Recoilless, 57 mm. Ml8 . 1-8
1-2 Rifle, Recoilless, 75 mm, M20 . 1-10
1-3 Rifle, Recoilless, 105 mm, M27; Jeep and Towed
Mounts. 1 -. 2
1 -4 Rifle, ilecoiiless, 106 mm, M40, on the Ground Mount. 1-15
1 -5 Rifle, Recoiliess, 90 mm, M67, With Cartridge, HEAT,
M371 (Sectioned). 1-21
1-6 Rifle, Recoilless, 90 mm, T234 . 1-23
1—7 Rear View of Recoilless Rifle, Repeating, 105 mm,
T189, Breech Open . 1-24
1-8 Sketch of T189 Rifle Modified fcr Gas Operation .... 1-25
1 -9 Revolver Type, Repeating Recoilless Rifle, T237,
Sectional Views. 1-27
1-10 105 mm Rifle, T136; 105 mm Mount, T149; Cal .50
Rifle, T43 and the Interim Sight. 1 -28
1-11 106 mm Self-propelled T165(ONTOS Vehicle). 1-29
1-12 120 mm Rifle, XM105 . 1-31
1-13 Recoilless Weapons-Conventional and Spigot Type ... 1-33
1-14 DAVY CROCKETT System, XM28, Man-portable,
2000-m Infantry Atomic Weapon. 1 -34
1- 15 DAVY CROCKETT XM29 Weapon System, 4000-m
Range . 1-35
2— 1 Schematic Functional Diagram Showing a Gun Back-
to-back With a Rocket iViotor To Achieve RecoiUess-
ness . 2-5
2—2 Schematic Recoilless Gun . 2-6
2—3 Gas Flow in the Chamber and Nozzle. 2-7
2-4 Rear Blast Danger Area of Rifle, 120 mm, AM105 .... 2-8
2-5 System Requirements. 2-10
2—6 Weight of Weapon vs Initial Energy of Projectiles for
Recoilless Systems . 2-11
2-7 Weight of Bare Rifle vs Energy, Momentum of Pro¬
jectile, for Recoilless Systems . 2-12
2- 8 Pressure vs Travel 120 mm HAW Recoilless Rifle . 2-14
2— 9 Bare Weapon Weight vs Peak Pressure .. 2-20
3— 1 Typical HEAT Recoilless Warhead Cross Section . 3-4
3-2 Typical HE Recoilless Warhead Cross Section. 3-5
3-3 Penetration as a Function of Projectile Spin Rate .... 3-8
3-4 Penetration for 30-deg Electroformed Copper Cones
into Mild Steel Targets . 3-10
3—5 Maximum Penetration into Mild Steel Targets at
Optimum Standoff vs Cone Angle for Electroformed
Cones . 3-11
3-6 Cone Thickness vs Penetration for 45-deg Copper
Cones . 3-13
xix
Preceding page blank
AMCP 70*338
LIST OF ILLUSTRATIONS (Cont'd)
Figure Title Page
.3-7 Fragment Mass Distribution . 3-17
3-8 Graphs for Determining the Initial Fragment Velocity V a 3-19
3- 9 Typical Angular Fragment Distribution ... 3-20
4- 1 Coordinate System. 4-8
4-2 Graph of l/s g vss d . 4-16
4—3 Projectile Shapes. 4—24
4-4 Drag Coefficient vs Mach Number . 4—25
4-5 Coordinate System for Trajectory Calculations . 4-28
4- 6 Angie of Elevation Nomogram . 4-31
5- 1 Schematic of Gun Showing Interior Ballistic Param¬
eters . 5-8
5-2 Weight of Propellant C t per Unit Projectile Weight M as
a Function of Muzzle Velocity V m for Ballistic Effi¬
ciencies ( e h - 0.4 and 0.5) . 5-14
5-3 (A) Chamber Volume as a Function of Propellant
Weight for Loading Densities 0.4,0.5, and 0.6
g-cnf 3 . 5-15
(B) Chamber Volume as a Function of Barrel Volume
(Bore Area Times Travel) for Expansion Ratios
2, 3,4, ard 5 . 5-15
5 -4 Muzzle Velocity as a Function of Projectile Travel in
the Barrel foi Peak Projectile Acceleration 2,500,
5,000, 7,500, and 10,000 g’s. 5-16
5-5 Charge to Projectile Weight Ratio as - Function of Re¬
duced Muzzle Velocity (V b K.A/A,) for Values of X from
C.3 to 0.6 . 5-20
5 -6 tp b (A/A t ) as a Function of V b l(A/A,) and X. 5-21
5—7 l(A/A f ) as a Function of Factor X . 5—22
5-8 ip'p as a Function of 4>‘ 0 and (\j/ b - V 0 )/Y b . 5-13
5-9 Bore Area Times Projectile Travel AL as a Function of
A Y and ij/j, V b . 5-24
5-10 Bore Area Times Projectile Travel AL as a Function of
AYmd4>' b V b . 5-25
5-11 Fffective Mass to Peak Pressure Ratio m'/P. as a
Function of AY for Values of 4>p X 10"* from 1 to 12. 5-26
5-12 Eff’ec l ive Mass to Peak Pressure Ratio m'/P p as a Func-
tior of AY for Values of tp' X 10" 4 from 1 to 20 .... 5-27
5-13 Charge to Projectile Weight Ratio C t /M as a Function
of Effective Projectile Mass to Projectile Weight Ratio
m '/iMfor Values of X from 0.3 to 0.6 .. 5-28
5-14 Ballistic Parameters as a Function of Factor X. 5-30
5-15 The Parameter 5 as a Function of the Projectiie Weight
to Charge Weight Ratio l '/Cj. 5-36
5-46 Burning Rate as a Function of Average Pressure for
M10 Composition Propellant, Lot FDAP81 . 5-38
xx
LIST OF ILLUSTRATIONS (Coat'd)
Figure Title Page
5—17 “Effective” Burning Rate Constant £ as a Func¬
tion of Maximum Pressure P p . S-40
5-18 /(K M ,X) as a Function of X. 5_53
5-19 ip p as a Function of 4>m . S-54
5-20 Equilibrium Temperature as a Function of Initial
Temperature Rise and Decay . 5_65
5—21 Number of Rounds To Achieve Given Fraction of
Equilibrium Temperature . 5-66
5-22 Reduced Temperature vs Round Number for Given
Rate of Fire ( h ' = 0.02 min -1 ) . 5_68
5-23 Reduced Temperature vs Round Number for Given
Rate of Fire (h' * 0.04 min -1 ) . S-69
5—24 Reduced Temperature vs Round Number for Given
Rate of Fire ( h’ * 0.06 min -1 ) . 5-70
5-25 Reduced Temperature vs Round Number for Given
Rate of Fire (A' ■ 0.08 min -1 ) . 5-71
5—26 Reduced Temperature vs Round Number for Given
Rate of Fire (h'“ 0.10 min -1 ) . 5-72
5-27 Reduced Temperature vs Round Number for Given
Rate of Fire (h ' = 0.12 min' 5 ) . 5-73
5—28 Reduced Temperature vs Round Number for Given
Rate of Fire (h'-0.14 min" 1 ) . 5-74
5-29 Reduced Temperature vs Round Number for Given
Rate of Fire (h ' - 0.16 min -11 ) . 5-75
5-30 Reduced Temperature vs Round Number for Given
Rate of Fire (A'* 0.18 min -1 ) . 5-76
5-31 Reduced Temperature vs Round Number for Given
Rate of Fire ( h ' = 0.20 min-') . 77
5-32 Experimental Temperature Distribution in Rifle. 5 ?9
5- 33 Multiplying Factor F 1 for Converting 7-perforated
Webs (IV 7 ) of M10 Propellant to Equivalent Single-
perforated Webs (W0 . S-S4
6- 1 Schematic of Nozzle Showing Design Parameters. 6-10
6-2 Distribution of Forces Acting on Nozzle. 6-13
6-3 Thrust Coefficient C F as a Function of Pressure Ratio
P 0 /Pe . 6-14
6—4 Calculated Optimum Thrust Coefficient C F as a Func¬
tion of Expansion Ratio e (7 = 1.3) . 6-15
6-5 Calculated Optimum Thrust Coefficient C F as a Func¬
tion of Expansion Ratio e( 7 s 1 . 2 ) 6-16
6-6 Chamber Pressure/Ideal Reservoir Pressure as a Func¬
tion of Chamber Area/Nozzle Throat Area (7 = 1.25) . 6-24
6-7 Lines of Constant Dimensionless Recoil « . 6-25
AkHCF 7M-23C
LIST OF ILLUSTRATIONS (Cont'd)
Figure Title Page
6-8 Percent Recoil Force Imbalance as a Function of
Nozzle Throat Area . 6-28
6-9 Effect of Approach Area A, on Recoil Imbalance of
the 57 mm Recoilless Rifle, M18 6-29
6-10 Theoretical Classification of Metals on the Basis of Heat
Transfer Properties . 6-36
6-! 1 Jet Boundaries for Jet Pressure Ratios from 1 to 10
(a = 5 deg, y= 1.2, A/ e = 2.0). 6-46
6-12 Jet Boundaries for Jet Pressure Ratios f ■om 1 to 10
(a = 10 deg, y = 1.2, M t = 2.0) . 6-46
6-13 Jet Boundary Patterns for Various Parameters . 6-47
6-14 Peak Pressure Contours for Backblast of the 105 mm
Recoilless Rifle, M27 . 6-50
6- 15 Typical Ducting Configurations . 6-52
7- 1 Comparison of Total Hit Probability p H for Different
Fire Control Systems as a Function of Range. 7-13
7-2 Probability of Hit-57 mm M18 Rifle;M306Al HE
Projectile . 7-13
7-3 Probability of Hit-57 mm M18 Rifle; M307 HEAT
Projectile . 7-14
7-4 Probability of Hit-75 mm M20 Rifle; M309 HE Pro¬
jectile . 7—14
7-5 Probability of Hit-75 mm M20 Rifle; M310 HEAT
Projectile . 7-15
7-6 Probability of Hit-105 mm M27 Rifle; M323 HE Pro¬
jectile . 7-15
7-7 Probability of Hit-105 mm M27 Rifle; M324 HEAT
Projectile . 7-16
7-8 Probability of Hit-90 mm M67 Rifle; M37I HEAT
Projectile . 7—16
7-9 Probability of Hit-106 mm M40 Rifle; M344 HEAT
Projectile . 7-19
7-10 Effect of Muzzle Velocity on Single Shot Hit Prob¬
ability . 7-20
7-11 Effect of Muzzle Velocity on Probability of One Hit
Out of Two Shots. 7-21
7-12 Effect of Muzzle Velocity on Probability of One Hit
Out of Three Shots . 7-22
7- 13 Variation of Expected Number of Kills With Range,
Caliber, and Muzzle Velocity . 7-26
8- 1 Velocity Measurement Schematic . 8-6
8-2 elocity Measurement With Staggered Array of Detec¬
tors . 8-6
8-3 Circuit for Breakwire System . 8-7
8—4 Measuring the Projectile Speed. 8—9
xxii
AMCP 706-231
LIST OF ILLUSTRATIONS (Cont'd)
Figure Title Page
8-5 Make System Circuit . 8-9
8-6 Solenoid Output Waveform. 8-10
8-7 Series Wiring of Coils . 8-11
8-8 Radar Velocity Measurement Schematic. 8-13
8-9 Radar Velocity and Displacement Schematic . 8-13
8-10 Radar Velocity and Displacement Schematic Using a
Reflector . 8-14
8-11 Simple X-band Interferometer . 8-14
8-12 Photographic Method for Velocity Measurement . 8-15
8-13 Copper Crusher Gage . 8-17
8-14 Input Circuit . 8-19
8-15 Strain Gage. 8-20
8-16 Divider Circuit for Strain Gage . 8-22
8-17 Portable Pendulum . 8-23
8-18 Photoelectric Recoil Measuring Device . 8-24
8-19 Bore-surface Thermocouple . 8-25
8-20 Typical Blast Waveform . 8-27
8- 21 Typical Pressure-Time Curve . 8-29
9- 1 The Davis Gun Mounted on WWI Martin Bomber .... 9-3
9-2 28 cm German Recoilless Gun . 9-4
9-3 Burney 95 mm R.C.L. Twin Jet Gun and Carriage .... 9- 6
9-4 60 mm Recoilless Mortar . 9-7
9-5 81 mm Recoilless Mortar . 9-8
9-6 4.2 in. Recoilless Chemical Mortar . 9-9
9-7 T135 Front Nozzle Rifle. 9-10
9-8 Side-loading Configuration . 9-11
9-9 Rifle, Repeating, 105 mm, T237, Assembly Drawing .. 9-12
9-10 Rifle, Repeating, 105 mm, T237, Installation Drawing . 9-13
9-11 Rifle, Repeating, 105 mm, T237, on Assembly Mount . 9-14
9-12 Configuration of Recoilless Rifle With Perforated Car¬
tridge Case . 9-16
9-13 M40 Rifle With Recoil Compensating Ring in Place ... 9-17
9-14 Sketch > Perforated Cartridge Case W ith Blow-out
Disc . 9-18
9- 15 Fin-stabilized Projectile With Propellant Attached,
Quarter Section-Ammunition, HEAT, 90 mm,
T249E6. 9-19
9-16 Hybrid Rocket-gun. 9-20
9-17 Configuration to Fire Over-caliber Projectile, DAVY
CROCKETT System, XM28 . 9-21
9-18 XM29 Weapon System Installed on M38A1 Vehicle .. 9-22
9-19 Rifle, Multiple, 106 mm. Self-propelled, M50. 9-24
9-20 M56, Type A, M40, Recoilless Rifle . 9-25
9-21 T114, Type B, Dual, M40 Recoilless Rifle . 9-26
LIST OF ILLUSTRATIONS (Cool'd)
Figure T/t'k Page
9-22 M50, Type A, Dual TP 7, Repeating Recoiiless
Rifle, Revolver Typ-- . 9-27
9-23 TJ14, Type B, Modified T237, Repeating Recoilless
Rifle, Revolver Type. 9-28
9-24 BB*1, (Mechanical Ram), Repeating Recoillest Rifle,
Magazine Type . 9-29
9-25 Ballistic Ram, Repeating Recoilless Rifle, Mounted on
Lightweight Vehicle . 9-30
9-26 M50, Type A, BB*1 (Mechanical Ram), Repeating Re-
coiiless Rifle, Magazine Type . 9-31
9- 27 MS6, Type B, BB-1 (Mechanical Ram), Repeating Re-
coilless Rifle, Magazine Type . 9-32
9-28 T114, Type B, BB-1 (Mechanical Ram), Repeating Re¬
coilless Rifle, Magazine Type . 9-33
9-29 T114, Type A, Ballistic Ram, Repeating Recoilless
Rifle, Magazine Type . 9-34
9-30 T! 14, Type B, Ballistic Ram, Repeating Recoilless
Rifle, Magazine Type . 9-35
9- 31 T114,Type B, Side-loading, Repeating Recoilless Rifle,
Magazine Type . 9-36
10- 1 antral Nozzle . 10-11
10-2 antral Nozzle With Bar. 10-12
10-3 antral Expanding Nozzle . 10-14
10-4 Multiple Nozzle and Front Orifice . 10-15
10-5 Annular Nozzle . 10-16
10-6 Interrupted Annular Nozzle . 10-16
10-7 Nozzle With Kidney-shaped Orifices . 10-17
10-8 Rotating Cam Ring Breech Mechanism . 10-21
10-9 Firing Mechanism . 10-30
10-10 Trigger Firing Mechanism, 120 mm Recoiiless Rifle,
XM105 . 10-32
10-11 Major Level of Actuation for Firing Mechanism, 120 mm
Recoilless Rifle. XM105 . 10-33
10-12 Minor Level of Actuation for Firing Mechanism, 120
mm Recoiiless Rifle, XMI0S. 10-34
10-13 Trigger Mechanism, 120 mm Recoiiless Rifle.XMlOS .. 10-35
10- 14 Trigger Mechanism Components, 120 mm Recoiiless
Rifle.XMlOS . 10-36
11- 1 106 mm Cartridge, HEAT, M344A1 . 11-4
11 -2 Folding Fin HEAT Projectile . 11-8
11 -3 Fixed Fin HEAT Projectile. 11-8
11-4 HE Projectile . 11-8
11-5 HEP Projectile. 11-8
11-6 WP Projectile . 11-9
11 -7 Canister Projectile . 11 -9
AMCP 706-23S
LIST OF ILLUSTRATIONS (Cont'd)
Figure Title Page
11-8 R -i*;ch Loading—Forward Orifice. 11-18
11 -9 Jhsech Loading-Rear Orifice, Perforated Case. 11-18
11-10 Axial Nozzle-Combustible Case. 11-18
11-11 Various Ballistic Parameters as a Function of a Percent
Perforation of Cartridge Case, M30A1B1 . 11-20
11-12 Various Ballistic Parameters as a Function of Cartridge
Case Hole Diameter . 11-21
11-13 Gas Flow Through Cartridge Case . 11-25
11-14 Pressure Differential Across 57 mm Cartridge Case,
M30 . 11-26
11-15 Perforation Array . 11-27
11-16 Perforated Case Force Diagram—Fixed Fins Conditions. 11 -29
11-17 Conventional Ignition System. 11-37
11-18 Linear (PYROCORE) Ignition System . 11-38
11- 19 P-T Curves for Good and Poor Ignition . 11 -46
12- 1 Prone Firing Position (Rifle, 57 mm, T66) . 12-2
12-2 106 mm Rifle Mount, M79. 12-3
12-3 106 mm Rifle Mount, T173 Removed from Tripod,
T26 . 12-4
12-4 Mount, XM124. 12-6
12-5 Two-hand Control (Tracking Handle and Trigger
Handle) Vehicle Mounted. 12-7
12-6 Integral Accessory Package for 90 mm Recoilless
Rifle, T234 . 12-8
12-7 Accessory Sleeve (Mounting Bracket) for HAW 120 mm
Rifie, XM 105 . 12-11
12-8 Ground and Vehicular Mount for 120 mm Recoilless
Rifle System, XM 105-HAW Jeep Mounted-Traveling
Position. 12-12
12-9 Exploded View of Telescope Mount, Ml 10 . 12-14
12-10 Rifle, Recoilless, 90 mm, M67 With 10 mm Pistol,
Spotting, XM14 and Telescope. 12-15
12- 11 Spotting Pistol, XM14 Mounted on 90 mm Rifle, M67 . 12-16
13- 1 Bullet, Spotter-tracer, Cal .50, M48A2 . 13-6
13-2 Design of Cal .50 Bullet, T140E12 ... . 13-8
XXV
AMCP 70^233
LIST OF TABLES
Table No. Title Page
I - 1 Data on Existing US Recoilless Rifles . 1-4
3- 1 Gumey Constant for Various Explosives. 3-18
4- 1 Recoilless Ammunition Characteristics . 4-22
5- 1 Ballistic Parameters for Several ^uns and Rounds. 5-9
5-2 Piezometric, Ballistic, and Th ,iodynamic Efficiencies
of Some Existing Recoilless Rifles . 5-13
5 -3(A) General Ballistic Design Data Based on Simplified Theory 5-17
5-3(B) Table of Parameters Based on Simplified
Theory . 5-19
5-4 Comparison of Theoretical and Observed Temperature
Data . 5-67
5- 5 Numerical Constants Used in Interior Ballistic Calcula¬
tions . 5-87
6- 1 Velocity Ratio and Expansion Ratio as Functions of
Pressure Ratio (7 = 1.23) . 6-13
6-2 Variation of Nozzle Thrust With Nozzle Expansion
Angle 2a . 6-27
6-3 Variation of Recoil Force Imbalance Vi ith Nozzle Ex¬
pansion Ratio . 6-27
6-4 Ballistic Data for the 57 mm Recoilless Rifle, M18,
Fired With Various Chamber Configurations. 6-30
6-5 Estimated Erosion of Gun Nozzles as a Function of
Bore Diameter . 6-38
6 - 6 Increase in Charge and Ammunition Weights-Y-duct
Compared With Case of Zero Included Angle . 6-53
7- 1 Magnitude of Errors for Calculating Hit Probability .. . 7-8
7-2 Magnitude of Mismatch System 1 . 7-10
7-3 Magnitude of Mismatch System 2. 7-11
7-4 Magnitude of Mismatch System 3 . 7-12
7-5 Single Shot Hit Probability-Visual Range Estimation .. 7-17
7-6 Single Shot Hit Probability-Crude Range Finder .... 7-17
7-7 Single Shot Hit Probability-Spotting Rifle. 7-18
7-8 Independent and Normally Distributed Quasi-combat
Errors Assumed To Cause Impact Errors . 7-19
9-1 Comparison of 75 mm Recoillcss and Clos< 1 Breech
Weapon Systems . 9-2
II- 1 Data for Some Recoilless Rifle Projectiles . 11-6
11-2 Cartridge Case Data for M30A1B1 . 11-23
11 -3 Composition of Several Propellants. 11-55
xxvii
Preceding page blank
AMCP 706236
PREFACE
The Engineering Design Handbook Series of the US Army Materiel
Command is a coordinated series of authoritative handbooks containing
basic information and fundamental data useful in the design and
development of Army materiel and systems so as to meet the tactical and the
technical needs of the Armed Forces.
This handbook, one of the Engineering Design Handbook Series, covers
the basic principles of operation of recoilless weapon systems, and provides
the fundamental design methods and procedures employed as a basis for
future design and development of such systems. Technologies and the
associated supporting scientific disciplines that are unique in application to
recoilless weapon systems are presented in sufficient detail tc provide the
design engineer with the system development rationale together with specific
subsystem design methodologies. Included in the presentation are highlights
of '.arly developments; system design and operation procedures; terminal,
exterior, and interior ballistics; recoil cancellation, system effectiveness, and
measurement techniques; basic design considerations; rifle and rifle
components; ammunition; mounts; and fire control. The extension and
adaptation of the basic technology to newer generation weapons-e.g., the
TOW System and DRAGON-are not covered.
This handbook was prepared by OEA, Inc., Des Plaines, Illinois—for the
Engineering Handbook Office, Research Triangle Institute, Research Triangle
Park, NC-under the general supervision of Mr. A. D. Kafadar; the principal
authois were Dr. Nuri Y. Olcer and Mr. Sam Levin. Technical coordination
was provided by an Ad Hoc Working Group—chaired by Mr. John J.
Donnelly, Frankford Arsenal-with representatives from the US Army
Tank-Automotive Command, Picatinny Arsenal, Rock Island Arsenal, and
Watervliet Arsenal.
The Engineering Design Handbooks fall into two basic categories-those
approved for release and sale, and those classified for security reasons. The
US Army Materiel Command policy L to release these Engineering Design
Handbooks in accordance with current DOD Directive 7230.7, dated 18
September 1973. All unclassified Handbooks can be obtained from the
National Technical Information Service (NTIS). Procedures for acquiring
these Handbooks follow:
a. All Department of Army activities having need for the Handbooks
must submit their request on an official requisition form (DA Form 17,
dated Jan 70) directly to:
xxix
Preceding page blank
S£TS
AMCP 706-236
JT *
■,j*
■,r.
,<&;•
4 j -
Commander
Letterkenny Army Depot
ATTN: AMXLE-ATD
Chambersburg, PA 17201
(Requests for classified documents must be submitted, with appropriate
“Need to Know” justification, to Letterkenny Army Depot.) DA activities
will not requisition Handbooks for further free distribution
b. All other requestors—DOD, Navy, Air Force, Marine Corps,
nonmilitary Government agencies, contractors, private industry, individuals,
universities, and others-must purchase these Handbooks from:
National Technical Information Service
Department of Commerce
Springfield, VA 22151
Classified documents may be released on a “Need to Know” basis verified by
an official Department of Army representative and processed from Defense
Documentation Center (DDC), ATTN: DDC-TSR, Cameron Station,
Alexandria, VA 22314.
Comments and suggestions on this Handbook are welcome and should be
addressed to:
Commander
US Army Materiel Development
and Readiness Command
ATTN: DRCRD-TT
Alexandria, VA 22333
(DA Forms 2028, Recommended Changes to Publications, which are
available through normal publications supply channels, may be used for
comments/suggestions.)
§
jK
XXX
AMCP 700-238
PART ONE
INTRODUCTION
CHAPTER 1
BACKGROUND INFORMATION
SECTION I
SCOPE
This handbook is an exposition of proven
methods and materials for the engineering
design of recoilless weapon systems. Its
purpose is to guide the engineer—the mature
practitioner as well as the novice-past known
pitfalls and more directly to his project goals.
By providing this comprehensive summary of
the available relevant technology and the
system engineering rationale, it is intended to
aid the technical manager, the project
engineer, and the component designer to
carry out his responsibilities with maximum
efficiency.
While the purpose of this handbook is to
give the engineer all the information he needs
to develop a complete system, greater stress is
laid on those principles and design features
unique to the recoilless weapon and ammuni¬
tion, avoiding unnecessary repetition of
material available in other Engineering Design
Handbooks and common texts. For example,
information that is obviously common to
other weapon subsystems-such as warhead
design, fuze design, optical sight design, and
ballistic measurements-are covered here only
in a general way to enable the engineer to
comprehend the intei.olationships of the
various subsystems in the context of the
integrated whole. References are given for the
detailed treatments of these areas that are
contained in the Engineering Design Hand¬
book Series and other pertinent documents.
This allows for more detailed and exhaustive
coverage of those aspects that are peculiar to
recoilless systems without excessive bulkiness
of the text. It is intended in this way to
maximize the utility of the handbook.
1-1
AM CP 706*238
SECTION II
HISTORY
1-1 GENERAL
The purpose of this section on history is to
summarize the work that has been accom¬
plished during recoilless weapon development.
This summary, while describing past achieve¬
ments, is intended to serve as a guide for
future work in this area. As a detailed history
would require several volumes, only the mujor
events in the program outlined u Table 1-1
are highlighted herein. In order for the reader
to obtain a more detailed description of these
programs, Refs. 26 and 27 provide an index
of the published recoilless rifle information.
1*2 HISTORY TO END OF WORLD WAR II
1*2.1 DEVELOPMENT PRIOR TO 1943
The idea of eliminating recoil from weapon
systems is not new. Leonardo da Vinci
(1452-1519), among the prodigious number
and variety cf mechanical concepts and
artistic works he endowed to mankind, left a
sketch of a recoilless gun concept showing
two projectiles fired simultaneously in oppo¬
site directions from a straight tube.
Work on the development of minimizing
recoil in guns has covered a little over one
hundred years, starting with the use of
pre-engraved, rotating bands as a means of
decreasing recoil being patented in 1857.
However, the modem history of recoilless
weapons does not begin until the beginning of
the twentieth century. In 1914, US Navy
Commander Cleland Davis developed the
concept of combining two guns back-to-back,
< firing the projectile forward and the other
firing a wad of grease and birdshot rearward
to yield net recoillessness. Commander Davis
obtained patents for this invention which he
reduced to practice and mounted experimen¬
tally on an airplane (Ref. 1).
In 1921, a British patent was issued to
Charles J. Cooke on a recoilless gun using the
vented propellant gas jet to balance the recoil.
However, the first recoilless guns developed
using this nozzle principle is accredited tc the
Russians. A Russian weapon of 76.2 mm
caliber was first introduced in 1936 and used
in combat in 1941 against the Finns. Its
design and construction were based on the
genual principles of a patent issued in 1917,
to the Russian mathematician, RiabouchinsVy
(Ref. 1).
The Germans developed a recoilless gun of
the two-projectile type in 1939 to equip
aircraft with large caliber (88 mm) armament
to attack surface targets. The recoil momen¬
tum imparted by firing its 7-kg projectile was
balanced by accelerating the cartridge case (of
equal weight) in the opposite direction. It was
mounted under the aircraft fuselage intended
for dive attack against battleships and other
important and difficult surface targets (Ref.
1 ).
Another German recoilless weapon, this
one constructed with a nozzle to use the gas
jet balancing principle, was issued to its field
forces in the early 1940’s for land combat. It
was the 75 mm L.G. 40. Also, in the early
1940’s, the British actively were investigating
recoilless guns with Sir Dennis Burney making
significant contributions to the advancement
of the technology.
1*2.2 DEVELOPMENT OF 57 mm RIFLE,
M18
US Army interest was aroused by the
knowledge of some of the foreign develop¬
ment and was stimulated by the prospect of
equipping the infantry with a lightweight
cannon capable of defeating armor. In early
1943, the US Army Chief of Ordnance
instructed one of its research and develop¬
ment elements, the Pitman-Dunn Laboratory
at Frankford Arsenal in Philadelphia, to
1-3
Preceding page blank
TABLE 1-1
DATA ON EXISTING US RECOILLESS RIFLES
Projectile _
Program
Dttigruiion
Caliber,
mm
Weight.
lb
Length,
in.
Type of Mount
Weight,
lb
Velocity,
fpe
Pressure,
psi
Start
Date
description
Status
Design Agency
TP?
37
49
Caliber .30 MG
1.81
1000
6,060
1346
Infantry shoulder
weapon
Shelved
Frankford
Arsenal
T1*(M18)
30
62
Caliber .30 . G
2.75
1200
6,500
1943
Infantry shoukter
or tripod wezpon
Standardized
Frankford Ar¬
senal, Frigi-
daire Division,
General Motors
Corp.
T66
57
27
48
Integral
2,75
1200
7,100
1947
Improved shoulder
Evaluation
Frankford Ar¬
or tripod weapon
senal
65
High-Performance
Shelved
Armour Re¬
Aircraft Rifle
search Founda¬
tion
T190
2.75 in.
Lightweight
launcher for air-
Develop¬
ment
Armour Re¬
search Founda¬
to-air
tion
716
75
67
65
Caliber 30 MG
6.75
1000
9,000
1943
Frankford Ar¬
senal
T17
75
250
86
Caliber .30 MG
14.00
1500
25,000
1944
National Forge
& Ord
Frankford Ar¬
senal
T21 (M20)
75
104
82
Caliber .30 MG
14.00
1000
9,000
1944
Infantry tripod
weapon
Standardized
Frankford Ar¬
senal
T192
75
3S
66
5.50
1000
5,550
1953
Scale model BAT
Develop¬
Frankford Ar¬
studies
ment
senal
TABLE 1-1
DATA ON EXISTING US RECOILLESS RIFLES (Continued)
Project* It
Program
Designation
Caliber,
mm
Weight
lb
Length,
in.
Type of Mount
Weight, Velocity,
lb fp*
Pressure,
psi
Start
Date
Description
Status
T41 (ARF)
75
75
68.75
Caliber .30 MG
-
-
-
1950
Lightweight semi¬
automatic shoulder
tripod
Develop¬
ment
T184
90
33
43.50
Integral
-
—
—
—
Platoon antitank
rocket launcher
(shoulder or tri¬
pod)
Develop¬
ment
T149
90
47
60
Integral
9.00
900
5,500
1948
Platoon antitank
rifle (shoulder or
tripod)
Develop¬
ment
T219E4 (M67)
90
35
53
Integral
6.80
700
7,780
1955
Platoon antitank
rifle (shoulder or
tripod)
Standardized
T234E
90
30.5
60
Integral
7.00
900
5,400
1955
Super-platoon
antitank rifle
(shoulder or tri¬
pod)
Shelved
T18
105
120
97
Caliber .30 MG
12.00
1000
9.000
1945
T19 |M27)
105
330
134
M22-M75
23.00
1250
10,000
1945
Jeep-mounted
rifle
Standardized
T135-7
105
307
-
T151
17.50
1660
16,600
1953
Front Nozzle,
battalion antitank
Develop¬
ment
rifle, mountable
cn ground, tri¬
pod, jeep, light-
armor vehicle
Design Agency
National Forge
& Ord, Armour
Research
Foundation
Midwest Re¬
search Insti¬
tute
A. D. Little,
Inc.
Midwest Re¬
search Insti¬
tute
United Shoe
Machinery
Frankford Ar¬
senal
Frankford Ar¬
senal
Frigidaire Divi¬
sion, General
Motors Corp.,
Armour Re¬
search
Foundation
TABLE 1-1
DATA ON EXISTING US RECOILLESS RIFLES (Continued)
Projectile
Designation
Caliber,
mm
Waight, Length,
jb in. Typu of Mount
Weight, Velocity,
lb fps
Pressure,
psi
Program
Start
Date
nfiAn
Status
T135
105
Semi-automatic
Develop¬
T136E2
105
240
134
T149
17.00
1700
10,400
1950
antitank rifle
Battalion antitank
rifle, mountable
on ground, tri¬
ment
Shelved
T137
105
pod, jeep
Battalion antitank
Develop¬
rifle, mountable
on ground, tripod,
jeep, light-armor
ment
T170E3
106
281
134
T149E3 (M79)
17.00
1650
10,400
1952
vehicle
Battalion antitank
rifle, mountable
Standardized
on ground, tripod,
jeep, light-armor
T237
105
820
144
-
17.00
-
—
1952
vehicle
Repeating antitank
Shelved
XM105E1
120
391
141.25
XM124
18.10
1810
9,100
1959
rifle
Heavy antitank
weapon, mount-
able on ground.
Shelved
XM63 (M28)
120
_
jeep
XM64 (M29)
155
_
M121
—
2,000-m range
Standardized
E1K
350
—
4,000-m range
Standardized
(or 8 in.)
—
1954
Long-range
Shelved
artillery rifle
Deign Agency
Harvey Ma¬
chine Company
Frankford Ar¬
senal
Firestone Tire
St Rubber Co.
Frankford
Arsenal
United Shoe
Machinery Co.
Frankford
Arsenal
Parish Pressed
Steel Co.
AMCP 706*238
)
explore the feasibility of .. applying the
recoilless principle to the development of a
man-portable, infantry-type weapon for de¬
feat of armor”. A program was established on
recoilless rifles under the general coordination
of Colonel Rene R. Studler, Assistant Chief of
Ordnance for Small Arms Research and
Development, and his staff, especially Dr.
Lafayette Boyd Hedge. Execution and the
technical direction of the program was
assigned to Frankford Arsenal. By mid-year,
Dr. William J. Kroeger, a physicist employed
in that laboratory, had evolved mathematical
expressions of the essential thermodynamic
relationships governing the ballistic operation
of recoilless guns. Concurrently, teaming up
with Mr. C. Walton Musser and a small group
of scientists and engineers, these principles
were reduced to practice in the form of an
experimental recoilless gun consisting of a
smooth-bore 2.75-in. caliber tube, a propel¬
lant combustion chamber, and a breechblock
perforated with many small nozzles. This fust
laboratory “test gun” was fired on 27 July
1943 (Ref. 2).
At a meeting held at the Office, Chief of
Ordnance, on 10 September 1943, it was
decided to center the first recoilless rifle
design about a caliber 57 mm shoulder-fired
rifle firing a 2.75-lb pre-engraved projectile.
Test Gun No. 2 was designed, but even before
test data from Gun No. 2 were available, the
demand for a lightweight weapon prompted
the start of the final design of the 57 mm
weapon. By October 1943, a firm practical
design of the 57 mm Rifle, T15 (Ml8) was
achieved.
The T15 Rifle proceeded through di /elop-
ment test firings, beginning in October 1943
at Frankford Arsenal and ending with a final
demonstration before War and Navy Depart¬
ment representatives on 26 September 1944
at Aberdeen Proving Ground. In early 1945,
limited production of the 57 mm rifle was
begun and in March 1945, a shipment of fifty
rifles was made for use in European Theater.
During April and May 1945, changes in
design, dictated by observation of weapons in
combat conditions, were initiated. The design
of the weapon then was felt to be adequate
for service use and at the request of the
Army Ground Forces, the 57 mm rifle was
standardized in June 1945 (Ref. 2). In less
than two years from the beginning of
development, the first standardized US
recoilless weapon system was issued to
combat troops. This was the Rifle, Recoilless,
57 mm, M18 shown in F ! g. 1-1. in 1945,
General Sornerve’J, Conr ending General of
the Army Service Forces, reporting to the
Congress on the development of new
weapons, stated: ‘Together with the V-T
fuze, the recoilless gun was the most startling
development of the War until the moment the
atomic bomb exploded” (Ref. 2).
The design goal of this system was one-man
portability and operability, with a second
man to resupply ammunition and load
subsequert rounds in a series. It was to be
fired from the shoulder standing or kneeling
and from the ground suppe :d on a light
integral mount. Its role was to give the
infancy heavy weapons platoon organic fire
support capability, complementing the mortar
and machine gun with fiat trajectory high
explosive firepower. The 57 mm M18 System,
weighing 44 lb, and designed to fire a 2.75-lb
projectile at 1200 fps, was used with striking
success in the European and Pacific Theaters
in WWII.
The unique design features that con¬
tributed principally to the successful culmina¬
tion of this project in a practical field weapon
were:
1 . Pre-engraved Rotating Band. This elimi¬
nated the engraving loads on the rifling and
the induced stresses in the gun tube,
permitting a thin-walled, lightweight tube
structure. Also, it eliminated the engraving
force irreproducibility and the resulting
feedback effects into the interior ballistics
and recoil balance.
2 . Perforated Cartridge Case. The cartridge
case was designed to perform the traditional
functions of containing the propellant,
projectile, and primer integrating the ammuni-
1-7
■zsiKssxcm&(mf^
tion as a package, and interfacing mechanical¬
ly with the gun structure. In addition,
however, the cartridge case served as a cage,
supporting the propellant during ignition and
venting the recoil-balancing gases through tjhe
case liner and then through the perforatiops
in its sidewall, allowing die gases to streap?
rearward along the chamber and to exit
through the nozzles.
3. Nozzle Design. The nozzle was designed
with helical cant to balance projectile spin
torque reaction and with simple nozzle
“blocks” to enable field adjustment of the
effective flow characteristics to restore recoil
balance after erosion of the nozzle had
progressed to an unacceptable level of recoil
imbalance.
4. Pressure Joints. The threaded joint
connecting the chamber and tube was
designed so that internal pressure tended to
force the mating parts more tightly together
due to elastic deformation during the ballistic
cycle. This has become a basic design tenet in
recoilless weapon engineering as well as in
other pressure vessel design. It is more
important in the thin wall sections found in
recoilless systems than in conventional guns
which have much thicker sections.
1-2.3 DEVELOPMENT OF 75 mm RIFLE,
T21 (M20)
Following a demonstration of the Rifle, 57
mm, T15, to the US Army Infantry Board in
February 1944, it was recommended by
Headquarters, Army Ground Forces that
weapons of the 75 mm and 105 mm size be
developed in addition to the 57 mm recoilless
rifle. Design requirements called for a 75 mm
size rifle that fired a projectile weighing
approximately 5 lb with a muzzle velocity of
1000 fps. Chamber pressure was to be
approximately 4000 psi. The design of the 75
mm size rifle was performed at Frankford
Arsenal and was given the nomenclature
“Rifle, 75 mm, T16”. The design of the Rifle,
75 mm, T16, was based on the principles
which contributed to the success of the initial
AMCP7M-2M
x «fro
Rifle, 57 mm, T15. However, the design
pxfteeded the original Ordnance Office direc¬
tives and it was recommended that: the T16
$psign project be clojied, but that further
consideration be given to a 75 mm, size rifle
fef.3).
cHv ;
A change in requirements to a 75 mm size
rifle firing a. (jopyentiojial 75 nun ,fl#illejy
project at a,yelacity of 1500 fg^ at^
chamber pressure of 2,S,0£)0 psi prompted the
design of a rifle designated ^gs ^igg,
Rccoilless, 75 mm, T17, The 7 gun was
designed to be fired electrically and use a
nonperforated cartridge case. The case fitted
the contour of the chamber and had its own
venturi. “Consequently”, as stated in Ref. 3,
“there, were no erosion problems such as
would have been encountered in a recoilless
rifle having the venturi as part of the breech
and firing at these pressures”. Changing
tactical requirements prompted the discon¬
tinuance of work on the T17 Rifle in favor of
a lightweight weapon that would fire a
standard HEAT projectile at a muzzle velocity
of 2150 fps and a chamber pressure of
approximately 7000 psi (Rifle, Recoilless, 75
mm, T21).
The final design configuration of the T? 1
Rifle was designated as Rifle, 75 mm, T21E4.
The T21E4 Rifle fired a 75 mm HEAT
projectile with a muzzle velocity of 1000 fps
at a chamber pressure of 6500 psi. The
complete weight of the rifle was 110 lb with
design features basically similar to those of
the Rifle, 57 mm, Ml8. A notable difference
was the interrupted thread breechblock,
annular nozzle, and tapered chamber of the
T21E4 Rifle. With respect to the p re-engraved
rotating band, cartridge case with perforated
sidewall, helically canted nozzle, and self-seal¬
ing joints, the T21E4 Rifle carried out the
basic design principles of the M18 incorporat¬
ing refinements based on the additional
experience gained. The T21E4 Rifle was
furnished to troops in the European Theater
in March 1945. Approximately three months
later, the T21E4 Rifle was standardized along
with Rifle, 57 mm, M18 and designated as
Rifle, 75 mm, M20 as shown in Fig. i-2.
1-9
AMCP 706-238
1-2.4 DEVELOPMENT OF 105 mm RIFLE
TO END OF WORLD WAR II
As stated in par. 1-2.3, the desire to have a
lightweight rifle in the 10S mm class also was
expressed after the successful demonstration
of the Rifle, 57 mm, T15. The task of
designing this weapon was assigned to
Frankford Arsenal. Performance requirements
were scaled from the T15 Rifle and indicated
that a lightweight projectile weighing approxi¬
mately 10 lb could be fired at a muzzle
velocity of 1000 fps at a rated maximum
chamber pressure of approximately 8000 psi.
Other requirements included the use of a
perforated cartridge case and the use of a
pre-engraved projectile in order to utilize best
the principles established during ballistic
experience with the T15 Rifle. Designated as
the T18 configuration, the initial 105 mm
rifle design was abandoned since it was felt
the time necessary to develop and manufac¬
ture the lightweight projectile was excessive.
Tliis design, therefore, was superseded by the
Rifle, 105 mm, T19, which fired a standard
projectile already being manufactured (Ref.
3).
The Rifle, 105 mm, T19 was designed to
fire a standard HEAT 105 mm projectile at a
velocity of 12S0 fps at a chamber pressure of
8500 psi. The rifle weighed approximately
352 lb and was comparable in performance to
the Howitzer, 105 mm, M2A1 with HEAT
Projectile, M67.
The designs of the firing and breech
mechanisms of the T18 and T19 Rifles are
similar to those of the Rifle. 75 mm, M20.
The only major difference is that the
interrupted thread lugs retaining the breech¬
block in place are integral with the chamber
in the Rifle T18; whereas, in the M20 Rifle
and TI9 Rifle, the lugs holding the
breechblock are contained in a bushing that is
in turn threaded into the chamber.
By April, 1944, just 6 months after
program inception, Frankford Arsenal had
completed the first of the TI9 models. In
May 1944, this T19 rifle was demonstrated
successfully before representatives from the
War Department General Staff, Headquarters
Army Ground Forces, and Service Boards.
However, active development of the 105 mm
rifle was suspended by Ordnance Committee
action in June 1947.
1-3 HISTORY POST-WORLD WAR II
1-3.1 DEVELOPMENT OF 105 mm RIFLE,
T13 (M27)
In February 1950, the 105 mm recoilless
rifle program was reactivated. Firing tests of
the T19 Rifle were continued and led to the
standardization of the T19 Rifle. Designated
as the Rifle, 105 mm, M27 (Fig. 1-3), the T19
Rifle became the interim standard 105 mm
recoilless rifle. It was used extensively and
with great success in the Korean action. In
addition, a program was initiated for the
development of a 105 mm battalion antitank
(BAT) weapon that would meet field
requirements more completely.
During the BAT program, a fixed fin-sta¬
bilized 105 mm HEAT round, capable of
defeating any known tank at a range of 1000
yd was being developed. Concurrently, it was
felt that a similar round should be developed
for the interim standard Rifle, 105 mm, M27.
The complete family of lightweight projectiles
for the M27 Rifle then would include the
fixed fin-stabilized 105 mm HEAT projectile
designated as the T184 along with the
spin-stabilized T268 HE (high explosive),
T269 WP (white phosphorus), and T139 HEP
(high explosive plastic) Projectiles.
With the development of a fixed fin-stabi¬
lized round, it was necessary to counterbore
the M27 Rifle in order for it to accept the 2
in. longer cartridge case. In addition to this
modification of the existing M27 Rifle and
the standardization of the 105 mm, T184
round, considerable work was performed in
trying to improve the long range flight
characteristics of the spin-stabilized HE and
WP projectiles. This work mainly was
concerned with substituting different types of
1-11
AKCP7M4M
Figure 1-3. Rifle, Recoilless, 105 mm, M27; Jeep and Towed Mounts
boattail bases for the existing round base.
1-&2 DEVELOPMENT OF 106 mm BAT
WEAPON SYSTEM
1-3.2.1 Development at Frankford Arsenal
In April 1950, Frankford Arsenal was
assigned to supervise the initial development
studies of a 105 mm battalion antitank (BAT)
weapon. By August 1950, Frankford Arsenal
was given the overall technical supervisory
role in the total BAT development project.
Besides this supervisory role, Frankford
Arsenal was to develop a long chamber 105
mm recoilless weapon (and mount) using the
rear nozzle principle and a smooth bore tube
to launch a long boom, fixed-fin projectile.
The rifle was designated as the T136 (Ref. 2).
Design of the barrel and chamber of the
T136 Rifle was completed by October 1950.
The first prototype barrel was assembled with
a chamber made at Frankford Arsenal and a
modified nozzle-breech assembly from a
Rifle, 105 mm, M27. The decision to use a
modified M27 breech was dictated largely by
the urgency of the development program.
M2
AMCP 706-298
This first T136 Rifle was proof-fired satisfac¬
torily in February 1951. The second and third
T136 Rifles were assembled during the spring
of 1951 and were equipped with mounts and
cal .50 spotting rifles. By completion of
development test firings with the T136 Rifles,
the following chief features had * een achieved
(Ref. 2):
1. At 195 lb, the rifle weighed 13S lb less
than the weight of the Rifle, 105 mm, M27.
2. The rifle had a smooth bore for use with
fin-stabilized projectiles.
3. The barrel incorporated the principle of
“strain compensation", developed in connec¬
tion with the Rifle, 57 mm, T66. Calculations
showed, in the case of a thin-walled rifle, that
the increase in the bore during firing
combined with the barrel and projectile
tolerances and the projectile clearance might
easily account for the yawing and consequent
observed inaccuracy. Performance was im¬
proved significantly by the use of oversize
projectiles which fir the barrel during rather
than before firing.
4. The breech opened from right to left to
permit easier loading when the loader stands
in his normal position on the right side of the
weapon. Moreover, the breech operating
handle could be placed either above or below
the chamber.
In October, 1951, certain design changes
were made. The resulting T136E1 Rifle
differed from the T136 model in the
following respects (Ref. 2):
1. Portions of the chamber and barrel were
increased in diameter in order to provide
clearance for cases fabricated from sheet.
2. The barrel was machined to accept the
plastic rotating band immediately ahead of
the oversized bourrelet of the 105 mm
T118E10 Projectile.
3. The barrel was provided with shallow
groove rifling (0.006 in. deep) with 1 turn in
360 calibers.
4. Both spotting rifle brackets were im¬
proved. The rear bracket was designed to
serve also as a mounting for the sights. (A
spotting rifle is a subcaliber weapon that fires
a projectile whose trajectory matches the
major caliber projectile (see par. 1-3.Z.5).)
•Soon after the T136E1 model was
designed, the need arose for more rapid firing,
up to 6 rounds per minute, which is about the
limit for manual operation. At this rate of
fire, the gun temperature soon rises to a point
where there is significant degradation in the
yield strength of the steel. These considera¬
tions led, in early 1952, to a 105 mm Rifle,
T136E2, which provided adequate strength
up to 600°F. The weight of this rifle was 214
lb as compared with the 197 lb of the
T136E1 model.
Development of major caliber ammunition
at Frankford Arsenal centered chiefly around
the T118 and the T184 designs. The T11S
round was intended to supply a fin-stabilized
projectile for use in the smooth bore T136
Rifle. It originally was scaled up from the 90
mm T108 round, which had a projectile with
a long boom and fixed-fins. The first design
had four fins that were shrouded, i.e.,
enclosed to an open end cylinder. Since this
shreud often was damaged during launching,
the design was changed to six longer and
unshrouded fins. This remained as the basic
design throughout the entire series of E
numbers, which differed chiefly in modifica¬
tions of the case and the liner. The only other
charge in the projectile (T118E10) consisted
of increasing the bourrelet diameter from
4.133 to 4.145 in. This increase was made in
order to take advantage of the principle of
strain compensation. The bourrelet, whose
diameter was 0.011 in. more than that of the
static bore, fit the strained bore. This
improved the obturation (prevention of gas
•JH
AMCP 706-238
jf..
; *■
h"
•■a^. :
*fi
(V
■ S
escape between bore and bourrelet) and
insured against excessive weapon wear and
erratic launching of the projectile from the
muzzle (Ref. 2).
The T184 round development was begun in
February 1951, and was intended to furnish
an improved HEAT round for interim use in
the M27 Rifle. It was fin-stabilized and
incorporated the latest fuze and shaped
charge design. It was almost identical with the
T118, but it did not have the oversize
bourrelet and it did have canted fins (4 deg)
in order to maintain a modest spin (approxi¬
mately 10 rev per sec) that appeared adequate
during flight and did not degrade terminal
penetration significantly. This was considered
necessary to smooth out the effects of small
aerodynamic asymmetries imparted by fric¬
tion in traversing the launcher. This round
was fired satisfactorily for accuracy in the
summer of 1951 (Ref. 2).
1-3.2.2 Development at Firestone
In August 1950, the Firestone Tire and
Rubber Company was awarded a contract for
the design and development of a short
chamber, 105 mm recoilless rifle (and
mount). This rifle was to have the rear nozzle
principle and be able to use either a slow spin
projectile fired from a rifled tube or a fin- or
drag-stabilized projectile launched from a
smooth bore tube. This gun was designated as
the T137. Firestone was also requested to
investigate a slow-spin round, conceived at the
Ballistic Research Laboratories and designated
as the T138 round; a folding-fin type round
(similar to a 75 mm round developed by
Armour Research Foundation) designated as
the Til 9; and a drag-stabilized or short-fixed
fin round, known as “Moby-Dick” and desig¬
nated as the T171 (Ref. 2).
In a three year period, 1950-1953,
Firestone developed the “short chamber”
(500 in? chamber volume) Recoilless Rifle
and Mount designated as the T!37 and T152,
respectively. Beginning with interior ballistic
calculations in September 1950, the first
prototype T137 Rifle was assembled and
proof tested by June 1951. The greatest
number of test firings were conducted with
the slow spin-stabilized T138 Projectile and
by the end of 1952, the round was developed
to the point of satisfactory accuracy at ranges
up to 1500 yd.
The folding-fin Projectile HEAT, T119 was
developed by mid-1953 (preferential treat¬
ment was given to the T138 Projectile as
requested) to the point of satisfactory
accuracy at 1000 yd from a smooth bore
tube. When the requirement from the ONTOS
(see par. 1-4.7) development specified a rifled
tube, Firestone altered the T119 Projectile
and secured a “better-than-required” accuracy
at ranges up to 2000 yd. In July 1953, the
T119E11 Projectile was approved as the
standard ammunition for the BAT Rifle and
Ammunition System and was designated as
“Shell, HEAT, 106 mm, M344” (Ref. 2).
1-3.2.3 Development of 106 mm Rifle, M4Q
The 105 mm, T136 Rifle System was
developed to a high degree of compliance
with the military characteristics desired by
the Army Field Forces; prototypes of weapon
and ammunition successfully passing engineer¬
ing te^.s at Aberdeen Proving Ground.
However, the ammunition design was based
on the concept of a family of homogeneous
fin-stabilized projectiles. During 1952, the
decision was made by the Ordnance Office to
include only one fin-stabilized projectile (the
HEAT) in the family; the balance being
spin-stabilized projectiles. In view of this
decision, the Arsenal project was re-oriented;
the development of the T136 system was
suspended and major attention concentrated
on the development of the T) 70 Rifle, which
had been initiated in the fall of 1951. It was
to have conventional deep rifling so th; t it
would be capable of firing spin- as well as
fin-stabilized projectiles. This rifle, whose
design was completed late in 1951, used
standard breech components of the M27
D
1-14
AM CP 706-238
RIFLE, 106 MILLIMETER: M40A1
W/RIFLE, CAUSER SO,
SPOTTING M8C
MOUNT, RIFLE* W6MM, M79
Figure 1-4. Rifle, Recoilfess, 106 mm, M40,
on the Ground Mount
Rifle, that were altered slightly to prevent
firing of similar but unsuitable ammunition. It
was similar in external appearance to the
T136 Rifle and incorporated the various
improvements that gradually had been added
to the design of the T136-such as high yield
strength material, strain compensation, spot¬
ting rifle mounting brackets, firing mech¬
anism, and breech opening mechanism.
Prototypes of the T170 barrel were made at
Frankford and Watervliet Arsenals. This rifle
was later standardized as the Rifle, Recoilless,
106 mm, M40 shown in Fig. 1-4 and became
the major caliber weapon of the BAT System
(Ref. 2). The M40 Rifle actually has a bore
diameter of 105 mm, but is called 106 mm
for logistical reasons.
In July 1952, the Office, Chief of
Ordnance decided that the Frankford Arsenal
T170 and Firestone T137 Systems would
remain as possible choices for the BAT Rifle
System. At this time, the following combina¬
tions of packages of projectiles were selected
for use in the BAT System:
1. T138E57 HEAT, T263 HE, T261 WP
fired from a tube with a twist of 1 turn in 200
calibers
2. T119 HEAT, M323 HE, M325 WP,
M326 (T139E36) HEP fired from a tube with
a rifling twist of 1 turn in 20 calibers
3. T184 HEAT, M323 HE, M325 WP,
M326 (T139E36) HEP fired from a tube with
a rifling twist of 1 turn in 20 calibers.
Combined engineering and field service
tests were performed at the Aberdeen Proving
Ground during September and October, 1952,
to evaluate the rifles and ammunition still
under consideration for the BAT System. As a
result of these tests, the T138 slow-spin
projectile was eliminated from further con¬
sideration because its performance at an
extended range of 2000 yd was inferior to
that of the finned HEAT rounds. As a result
of subsequent Service Board Tests at Fort
Benning, Georgia, and further comparison
tests at Aberdeen Proving Ground, the
Frankford Arsenal T170 Rifle and the
Firestone Tire and Rubber Company Car¬
tridge Til 9E11 were selected for the interim
BAT System. Upon standardization, the
HEAT folding-fin-stabilized cartridge was
designated as M344, and the T170 Rifle
designated as the Rifle, 106 mm, M40.
1-3.2.4 Development at Frigidaire
The Frigidaire Division of the General
Motors Corporation was assigned the task of
developing a front-orifice type 105 mm
recoilless rifle for the battalion antitank
(BAT) Weapon System. This rifle was to be
capable of firing the projectiles developed
under the other BAT programs. Designated as
T135, the front-orifice rifle used a solid-wall
cartridge case instead of a perforated case.
The exit of the recoil neutralizing propellant
gases took place at the mouth of the cartridge
case, expanding radially into a small chamber
and then rearward through the nozzle.
Specifications called for the BAT Rifle to
fire a 17.5-lb projectile 1000 yd with a 0.25
mil accuracy. The maximum chamber pres¬
sure was to be about 12,500 psi with a muzzle
velocity of 1750 fps. The desired weight of
the rifle was to be as near 200 lb as possible.
Interior ballistics were performed by the
Armour Research r undation in order to
1-15
AMCP 706-238
establish the chamber, barrel length, and
nozzle characteristics of the first prototype
rifle. The design of the T13S Rifle evolved
through seven prototype and engineering
models with firing tests of the various models
performed at Fort Sheridan, Illinois, by
Armour Research Foundation. Although the
final models of the T13S weapon were found
to be mechanically and ballistically successful,
the lightest model weighed over 300 lb. With
studies indicating that the minimum possible
weight for a front-orifice rifle would be
250-275 lb, it was decided that the front-ori¬
fice rifle was too heavy to be carried by hand.
As a result, the program was terminated with
the manufacture of the last engineering mod¬
el.
1-3.2.5 Spotting Rifle Development
During the formulation of the BAT
program, the concept of the subcaliber
spotting rifle was introduced. The subcaliber
rifle is mounted on the major caliber rifle
with its bore approximately parallel to that of
the major caliber rifle in order for the
spotting projectile to match the trajectory of
the major caliber round. In May 1950,
Frankford Arsenal was advised by the Office,
Chief of Ordnance that it should initiate work
on the design of a cal .50 spotting bullet
having as high a ballistic coefficient as
possible. By July 1950, the development of a
cal .50 spotting rifle was begun at both
Springfield Armory and the Remington Anns
Company.
The development of the cal .50 spotting
rifle at Springfield Armory resulted in four
distinct rifle models. These four models were
designated as the T43, T46, T46E1, and
T46E2. The first of these models, designated
as the T43, used the standard cal .50 machine
gun cartridge loaded with a reduced powder
charge. Weighing 661 grains, the projectile
attained a muzzle velocity of 1800 fps in a
36-in. length barrel. The T43 Rifle was
semiautomatic, gas-operated, and used a
conventional double column box magazine.
Due to the urgency of the spotting rifle
development, detailed component drawings of
the T43 model were prepared in haste, the
first component drawings being released for
fabrication just nine weeks after program
initiation. The first test T43 model was fired
at Springfield Armory 17 weeks after the
program start. Since the drawings were
hurriedly prepared, they did not guarantee
100 percent interchangeability of parts for
assembly and function, and because the
service life of the component parts left much
to be desired, only limited test firings were
conducted with the T43 Rifle (Ref. 4).
The second step in spotting rifle develop¬
ment was undertaken to correct deficiencies
existing in the previous T43 model and to
accommodate a shortened cal .50 round. The
length of the round was reduced because of
the reduced powder charge required. The
major design change in the T46 Rifle was a
change in the slide and receiver design from a
rectangular to cylindrical construction while
retaining the rectangular bolt construction of
the T43 Rifle. While better than the T43
design, the T46 Rifle was plagued with the
following types of malfunctions: (1) operat¬
ing power was marginal, (2) ammunition was
not properly fed, (3) spent cartridge cases
were not always ejected, and (4) firing
mechanism failed to remain in the cocked or
ready-to-fire position.
In order to eliminate these malfunctions
and reduce the firing error of the T46 model,
a second redesign of the cal .50 spotting rifle
was made. Designated as the T46E1, the new
model had a reduced barrel length, from 36 to
32 in., in order to reduce the projectile travel
from gas port to muzzle exit. This change
causes an earlier venting of the bore gas
pressure, resulting in a lower chamber
pressure at the instant of breech unlock. An
increase in the initial volume of the gas
system retarded the early acceleration of the
piston and, because of the resulting longer gas
system time, the maximum piston velocity is
reduced.
116
AMCP 706-238
While these design changes led to improve¬
ments in the weapon accuracy and a
reduction in the number of malfunctions, it
was felt that a better design was still in order.
In the next redesign of the spotting rifle,
designated as the T46E2, three major
improvements were made:
1. The cable pull load required to fire the
rifle was reduced.
2. The rebound of the operating slide from
battery was reduced.
3. The operating power in the rifle was
controlled by incorporation of a needle-valve
type gas-regulator. The improvement of the
spring forces acting on the firing cable was a
matter of refinement and adjustment of the
existing design with the reduction of the slide
rebound, malfunctions caused by the hammer
impacting the firing pin when it is in the
locked, out-of-battery position were pre¬
vented. Differences in the effective rigidity of
the rifle mounts in addition to variation in the
magnitude and operating power given by
different lots of spotting rifle ammunition
made the incorporation of a power regulating
device imperative.
As a result of these design changes and
numerous refinements throughout the spot¬
ting rifle design, function was improved to a
substantially satisfactory status. The firing
error of the T46E2 was almost one-half the
error obtained with the T46 design; at a range
of 100 yd the mean target radius was 1.50 in.
for the T46E2 Rifle as compared to 2.82 in.
for the T46 model.
1-4 OTHER RECOILLESS WEAPONS OF
CALIBER 106 mm OR SMALLER
1-4.1 37 mm RIFLE, T62
In July, 1945, the Office, Chief of
Ordnance requested Frankford Arsenal Ord¬
nance Laboratory to design and develop a 37
mm single shot recoilless rifle. Designated as
the Rifle, 37 mm. T62, this rifle was to fire
Projectiles M54 and M63 at velocities of 1250
and 1200 fps, respectively. This 37 mm
recoilless rifle was to be shoulder-fired for use
as an antipersonnel type weapon. The first
design of the T62 Rifle was proof-fired in
May 1946. After firing of the tenth round,
the lugs that locked die breech into the
chamber showed evidence of failure by
bending and firing was discontinued. In
addition to the breech design failure, the first
eight rounds fired resulted in ignition failures
(Ref. 3).
The breech locking mechanism and the
firing mechanism were redesigned and the
new rifle model designated as T62E1. This
rifle functioned satisfactorily in subsequent
proof-firings. However, the charge, develop¬
ment to give the required bdBwtffc perfor¬
mance was never completed since- the project
was shelved.
1-4.2 67 mm RIFLE, T66
In 1951, a replacement of the original 57
mm recoilless rifle (the Ml8) was proposed.
Technological advances from those crude
models of relatively early days offered
improvements in practically all areas: interior
ballistics, flight dynamics, HEAT penetration
performance, and structural and mechanical
design of the rifle. The new rifle, designated
as the T66, was to match the performance of
the M18 Rifle (1200 fps) at a considerably
lower maximum chamber pressure. One of the
first considerations in the T66 barrel design
was the use of steel with a higher yield
strength. As such, the barrel walls could be of
relatively thin cross section as compared to
that of the M18 Rifle. By use of better
propellants, the first T66 Rifle design was 14
in. shorter than the M18 Rifle and weighed 28
lb including all accessories, as compared to a
weight of 44 lb for the M18.
All development activity proceeded
smoothly until firing tests were performed on
the initial T66 designs. Considerable in¬
accuracy was encountered and the basic cause
1-17
wwmwu i *
AMCP 706-238
was not known. After considerable investiga¬
tion into such possible causative areas as the
method of mounting and nozzle symmetry, it
was found that the projectiles were yawing
excessively. Upon further studies, it was
hypothesized that the yaw in exterior flight
was related to the yaw and balloting of the
projectile during bore travel. Further calcula¬
tions showed that the expansion of the highly
stressed barrel during firing could be great
enough so that if the projectile was crowded
to one side of the bore, it could become
disengaged completely from the rifling on the
other side. The use of high-speed X-ray
equipment verified this hypothesis by indicat¬
ing that significant yaw did occur within the
barrel (Ref. 6).
As a result of these studies, it was found
that for any weapon which uses a barrel that
is strained highly by firing, the projectile
should be designed to fit the barrel during the
firing rather than prior to firing. The use of
this strain compensation principle was first
made in the T66 Rifle and has been used on
many of the subsequent recoilless rifle
programs.
In late 1954, a requirement that the T66
Rifle be capable of firing the same mixed
family of fin- and spin-stabilized projectiles as
used in the M18 Rifle was added. The
projectiles developed during the T66 program
were the spin-stabilized HE, T115 and the
fixed fin-stabilized HEAT, T1S8. Complete
prototype systems were designed, built, and
successfully demonstrated. However, the
project was terminated due to lack of
sufficient user interest in 1958, after comple¬
tion of User Tests by the US Army Infantry
Board. More information on the T66
development program is found in Refs. 6, 7,
8, and 9.
1-4.3 2.75-in. RIFLE, T190
Among the programs conducted at the
Armour Research Foundation was the devel¬
opment of a single shot and repeating
recoilless rifle to fire the 2.75 in. boosted
rocket used in the T131 ammunition. These
weapons were intended for air-to-air use and,
accordingly, carried restrictions of light¬
weight, simplicity, and ease of loading.
Requirements for the single shot 2.7S-in.
recoilless rifle, designated as the T190, were
(Ref. 4).
1. F re a 2.75-in. spin-stabilized boosted
rocket with a nominal weight of 5.5 lb
2. A muzzle velocity of 1200 fps
3. Peak chamber pressure under 6000 psi if
possible
4. Recoil balance such that the transmitted
force to airframe is less than 1000 lb
5. Rifle front profile as small as possible
for purpose of parallel stacking of rifle
6. Use of muzzle and nozzle blast tubes to
shield adjacent portions of assembly and
aircraft
7. Weight of loaded cluster to be 300 lb
maximum.
The repeating version, designated as the T191,
has essentially the same requirements except
that the boosted rocket was to be fired in
automatic operation at a minimum rate of fire
of 600 rounds per minute.
The principal technical problems of interest
concerned design for minimum system
weight, interior ballistic uniformity at the low
operating pressure of 6000 psi and over the
extreme ambient temperature range foi
aircraft, automatic feed mechanism design,
effects of nozzle erosion, gun heating, and
blast effects on the aircraft structure.
Compromise solutions to the trunnion reac¬
tion problem were examined, considering
partial recoillessness coupled with soft mount¬
ing. Pre-engraved vs self-engraving rotating
bands in the uniform twist and increasing
twist were investigated with respect to
ballistic reproducibility. The pre-engraved
1-18
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TV"^f^r w» .'»»■■• i,*L 1
************ ** *m
AMCP 706*238
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band with uniform twist solution was found
to be superior. Perhaps the most striking
event in the program, from a technical history
standpoint, was the construction and testing
of a glass fiber reinforced plastic gun. In
collaboration with the US Naval Ordnance
Laboratory at White Oak, Maryland, a test
gun was designed, built, and fired (19S6). The
tube survived several shots at full pressure and
velocity, and showed acceptable ballistic
uniformity and projectile accuracy. This was
an early indication (perhaps the first) of the
potential for this material for construction of
guns. The project was terminated in 1957
with the suspension of the user requirement.
For more information regarding the 2.75-in.
recoilless rifle, the reader is directed to the
material found in Refs. 4 and 5.
14.4 90 mm RIFLE AND AMMUNITION
Various evaluation studies of multipurpose
shoulder weapons conducted in the late
1940's indicated the need for a 90 mm
recoilless rifle. As a result of these studies,
Arthur D. Little, Inc. and Midwest Research
Institute (MRI) were given Ordnance Con¬
tracts to develop 90 mm platoon antitank
(PAT) recoilless rifles. The objectives of these
programs were to develop a rifle weighing less
than 30 lb, capable of being either shoulder-
or ground-fired. The rifle was to have very
good accuracy at 500 yd and capable of
defeating tank armor up to 6 in. thick at a
maximum obliquity of 60 deg (Ref. 4).
The Arthur D. Little rifle, designated as the
T149, differed from the more conventional
rifles developed before 1951, being very light
for its caliber and using a novel breech
mechanism. The annular two-lobe nozzle
breech is basically a rotating cam ring which
locks the round in place, cocks the firing
mechanism, and actuates the extractor. Recoil
compensation is provided as the discharging
propellant gases pass through the annular
nozzle. The nozzle contours are formed by
the inner surfaces of the chamber and cam
ring, and by the outer surface of the cartridge
case base. Another unique feature of the
T149 Rifle was a firing mechanism in the
lower rear of the chamber which caused a
firing pin to be driven radially into a side-fire
percussion cap in the cartridge case base (Ref.
10 ).
The PAT program at Arthur D. Little also
included the development of fin-stabilized
HEAT, HE, and WP projectiles for use with
the T149 Rifle. Weighing 9 lb, the Projectile,
HEAT, T249 was the only projectile carried
through complete development. The design of
the HEAT projectile was based largely on t*»e
configuration of the 105 mm, Til8 con¬
figuration. One of the interesting and unique
design aspects of the T249 Projectile con¬
cerned the application of the rotating bands
to the projectile. Prior to PAT development,
rotating bands, whether plastic or metal, were
fabricated from sheet stock and then
cemented or brazed to the projectile, or
machined integrally from the projectile. In
the Projectile, HEAT, T249, a plastic rotating
band is injection-molded directly to the
projectile.
In February, 1952, the Office, Chief of
Ordnance, set priorities on the development
of a suitable PAT rifle-ammunition system.
Since the T149 Rifle was given last priority,
the T149 Rifle and ammunition were never
presented for user tests (Ref. 11).
Midwest Research Institute received its
PAT weapon contract in December 1951. The
objectives of the PAT program were very
similar to those given to Arthur D. Little. The
MRI 90 mm recoilless rifle design was
designated as the T184. The T184 Rifle
design also achieved a very low weight due to
the employment of two unconventional
features not previously used in recoilless rifle
design. The first unique aspect was the use of
a reverse tapered cartridge case (smaller in
diameter at die base than at the mouth). The
cartridge case has a 57 mm base which
permits the use of a reduced diameter and
thus lighter breechblock. Secondly, the
1-19
AMCP 706-206
breechblock was a two part assembly
composed of a steel breechblock and an
aluminum venturi expansion cone which
further contributed to weight reduction (Ref.
4).
Initial analytical studies in the PAT
program indicated that a conventional 64b
projectile and a recoilless rifle having a bare
weight of 2 5 lb could not meet the required
first round hit probability at the spedfied
range and muzzle velocity. Further studies
indicated the need for using rocket-assisted
(RA) projectiles. Two RA projectiles were
designed for use in the T184 Rifle. The first
projectile, designated as the T273 HEP RA,
was designed on the basis of a scaleup of the
2.75-in. T131 Rocket, and contained a solid
cast grain rocket motor and an HEP warhead.
The second projectile, designated as the T274
HEAT RA, employed the same rocket motor
as the T273 round but contained a shaped
charge warhead. Hie warhead was separated
from the rocket motor by a bearing section
that permitted the rocket motor to rotate at
the high speed required for round stability
while the warhead was maintained at the low
spin rate necessary for maximum terminal
effectiveness (Ref. 4).
As a result of the HEP round not being able
to meet the armor defeating requirements and
because of the wide dispersions encountered
during range firings, the T184 program was
terminated in 1955 in favor of the T219
program which was to be conducted at
Midwest Research Institute. The T219 PAT
Rifle requirements permitted an increase in
rifle weight to 30 lb so that the peak chamber
pressure could be increased in order to
eliminate the need for a rocket-assisted
projectile.
The 90 mm T219E4 model, as shown in
Fig. 1-5, is a 354b bore-sized, shoulder-fired,
recoilless rifle. The T219E4 PAT Rifle
incorporates a central-orifice, bar breech, and
may be loaded and fired by one man,
although it was designed for a two-man team.
Firing the Cartridge, 90 mm, HEAT, T249E6
at a muzzle velocity of 700 fps, the T219E4
Rifle attained a first round hit probability of
50 percent at 500 yd (unaided visual ranging)
and was capable of defeating any armor likely
to be encountered in the battle area. Primarily
designed as an antitank weapon, it was also
highly effective against emplacements and
grouped personnel.
By August 1959, the T219E4 Rifle,
T249E6 Cartridge, and auxiliary items were
standardized and designated as Rifle MAW
(medium antitank weapon), 90 mm, M67;
Cartridge, 90 mm, HEAT, M371; Telescope,
Ml03; and Telescope Mpunt, M110. This
standardization was given conditionally on
the basis that certain design corrections would
be made. The deficiency to be corrected was
the low temperature firing performance.
During arctic firing tests, the CARDE (see
par. 1-6.10) T31 sheet Propellant used in the
T249 Cartridge revealed an apparent tendency
toward high velocity levels and dispersions,
accompanied by potentially unsafe high
pressures. As a result, emphasis was placed in
establishing the suitability of a granular
propellant. On the basis of the best uniform
performance, MS Propellant was selected for
use in the M371 Cartridge. Other improve¬
ments made in the 90 mm MAW Weapon
System were the simplification of the breech
design, by reducing the number of compo¬
nents from 28 to 14 parts, and the
strengthening of the projectile spike-to-body
assembly in order to prevent separation of the
spike from the body.
During studies of the various designs for
the ultimate battalion antitank (U-BAT)
weapon, it became evident that one of the
designs offered greater promise in obtaining a
very lightweight rifle, and that a rifle of this
nature would find tremendous application in
the 90 mm PAT Rifle program. Designated a;
the T234, this rifle design had several unique
features. One feature was the absence of a
breech that must be opened and closed by the
operator. As described in Ref. 12, the nozzle
1-20
1-21
9U10L40IKV
AMCP 706-233
is of central-orifice design, but divided into
eight segments. The segments are spring-
loaded to the closed position so that as the
round is inserted, the nozzle moves forward
and expands radially outward to permit
chambering of a round with a larger diameter
than the nozzle throat.
Ammunition for the T234 Rifle was also
unique in that a metal cartridge case was not
used. Instead, a thin, plastic powder envelope,
which is consumed during firing, was used and
the need for expended case extraction
eliminated. Firing of the chambered round
was accomplished by attaching an ignition
transmission line (pigtail) to the projectile
boom. After chambering of the round, the
end of the pigtail is inserted into the firing
mechanism contained in the rear part of the
rifle mount. Shown in Fig. 1-6, the T234
Rifle weighs 34.S lb with the round weighing
8.5 lb.
In early 19f , several original requirements
were changed under the heading of the
Super-PAT program. The 90 mm Super-PAT
Rifle was to be essentially the same design as
the Inst configuration of the T234 Rifle.
However, it was to employ a high strength
steel barrel-chamber (200k to 205k psi yield
strength as compared to normal yield
strengdis of 160k to 175k psi). Internal,
external, and terminal ballistic conditions
were to remain the same, although with the
addition of cal .405 spotting pistol, the
weapon system was to be capable of a 90
percent first round hit probability at 500 yd.
In June 1958, the Super-PAT program was
terminated before completion of the develop¬
ment program as the major emphasis was
shifted to the T219 PAT program.
1-4.5 DEVELOPMENT OF REPEATING
RIFLES 105 mm, T189 AND T237
Beginning with German efforts during
World War II, there was a recurrent interest in
recoilless weapons capable of automatic or
semiautomatic fire. For combat vehicles (air,
ground, or water), the advantages of increased
rate of fire and in crew protection are quite
obvious. Some of the technical problems, too,
are quite obvious, including space limitation
at breech location, generally heavier ammuni¬
tion with a longer envelope, and the greater
penalty of mechanism weight in a weapon
system whose great attraction is light weight.
US efforts in repeating recoilless rifle
design were aimed principally toward applica¬
tions for land combat vehicles. Design of a
repeating 105 mm recoilless rifle began at the
United Shoe Machinery Corporation in 1953.
Designated as the T189 and shown in Fig. 1-7,
this repeating 105 mm recoilless rifle was
designed in both electrically and gas-operated
versions. As shown in Fig. 1-7, the electrically
operated T189 Rifle uses small electric gear
motors to rotate a five-round reel between the
barrel and breechblock. At each position of
the reel assembly, the barrel and breechblock
are coupled with a chamber containing one
round. The electrical drive system extends or
retracts and rotates the breechblock to open
or close and lock the round in place.
One of the designs for the gas-operated
T189 Rifle is shown schematically in Fig. 1-8
and operated in the following manner as
described in Ref. 13:
1. Energy obtained by bleeding propellant
gas from the barrel during the blow-down
period (i.e., the period after projectile exit
during which the internal pressure decays to
atmospheric) is used to compress a pair of
nested driving springs which then provide the
eneigy necessary to drive the indexing
mechanism through one cycle.
2. Gas is bled from a barrel port (shown in
Fig. 1-8) dose to the muzzle, which does not
become operative until the projectile has
passed. Upon projectile exit, the gas turning
into the port expands and is conveyed to the
cylinder through a telescoping length of
tubing. The gas flow into the cylinder lasts for
about 0.012 sec of the blow-down period. At
1-22
1-24
ing, 105 mm, T189, Breech Open'
AMCP 70*238
the end of this time, the blow-down pressure
and the pressure in the cylinder will reach
equilibrium. The quick acting check valve
then closes and retains the cylinder charge.
3. During the cylinder charging stroke, the
drive springs are compressed. The gear
segment, connected to the piston by means of
an operating link, is rotated and, in turn,
causes rotation of a pinion gear. The
movement of the gear segment is enough to
rotate the pinion gear backward just one turn.
The gun indexing cycle occurs on the return
stroke of the piston. The gas charge then is
allowed to escape through an exhaust valve
that is separate from the gas feed duct. When
the back pressure (acting on the piston) falls
below the driving spring pressure, the return
stroke begins. On the return stroke, the
pinion gear, now engaged to the gun camshaft
by the pawl, is rotated one turn forward,
indexing the gun through its cycle.
During efforts to simplify the operation
and design of the T189 Rifle, an entirely new
design was accomplished. It was decided that
the new design should replace the T189
design and was given the T237 designation.
The repeating T237 Rifle design incor¬
porated several design features. As shown in
Fig. 1-9, a drive motor was geared down to a
“lead screw" type drive shaft. Partial rotation
of this screw unlocked both the breechblock
and the barrel, and proceeded to translate the
breechblock axially a predetermined distance.
During breechblock travel, the reel containing
the five chambers also was moved rearward at
a slower rate so as to clear the end of the
barrel. A drum then revolves the reel into the
next position. When the electric motor is
reversed, the breechblock and reel are
returned to the closed position and rotational-
ly locked. The T237 repeating recoilless rifle
program proceeded through the manufacture
of one test rifle. However, the program
terminated in mid-1956 by the Office, Chief
of Ordnance before any extensive testing of
the system was performed.
1-4.6 DEVELOPMENT OF 105 mm RIFLE.
T136
The 10S mm Recoilless Rifle, T136 was
one of the three original weapon concepts
selected for development and evaluation
under Ordnance Project TS4-4024. Fig. 1-10
shows this weapon with T149 Mount and
with the cal .50 Rifle, T43 installed. In
October 1951, changes were incorporated
altering the configuration to the T136E1. In
November 1951, the design again was changed
where provision was included for the
expected 21 percent degradation in yield
strength of the gun steel when heated to
600° F. T136E2 was the designation assigned
to this temperature compensated recoilless
rifle.
Portions of this T13f system ultimately
were incorporated in the standardized BAT
Weapon, M40. The chamber design of tire
T136 was united with the barrel of the M27
to form the T170 configuration, which
ultimately became the standardized M40.
1-4.7 DEVELOPMENT OF WEAPON SYS¬
TEM T165 AND T166, SELF-PRO¬
PELLED (ONTOS) USING 106 mm,
T170 RECOILLESS RIFLE
The initial long range military characteris¬
tics of the BAT weapon system called for a
fully man-portable weapon weighing in the
neighborhood of 200 lb. The weapon that was
eventually standardized for the BAT program
was the Rifle, 106 mm, T170 (M40).
Weighing a total of 485 lb, the weapon could
be moved into position by a crew of two men
cr could be broken down for hand cany' by a
larger group. This was still a long way from
meeting the fully man-portable requirements.
As tire best interim carrier, the Jeep
Carrier, M38A1 provided:
1. A low silhouette
2. Good road and cross speed
1-26
AMCP 706-238
Figure 1-10 L 705 mm Rifle, 7735; 755 mm Mount, T149; Cat .50 Rifle,
T43 and the interim Sight
3. Ruggedness and reliability
4. Standardized simple operation and
maintenance
5. Good cruising ranges
6. Relatively low cost and ease of produc¬
tion.
However, it lacked:
1. Armor protection
2. Adequate load capacity
3. Cross-country mobility of a tracked
vehicle.
As a result, studies and searches were made
into the development of a lightweight
fully-tracked system that would be sufficient¬
ly durable and easily maintained for use in the
infantry regiment. Two systems were devel¬
oped around the (ONTOS) vehicle. A six-rifle,
ONTOS vehicle designated as the T166 shown
in Fig. 1-11 and a one-rifle ONTOS vehicle
designated as the T165 were developed using
Rifles, 106 mm, T170 similar to the
standardized M40 Rifle. This T165 system
was designed to go into combat with the six
BAT systems loaded and operable by the crew
within the protected compartment. It could
be fired in salvo, ripple, or single shot. After
firing, the breeches could be opened mechan¬
ically from within the vehicle, but subsequent
rounds were chambered by the crew reaching
out through the open doors. Two of the six
rifles were designed to be dismounted quickly
from the vehicle and operated from a
compact folding ground tripod when the
tactical situation required it. In service tests
of these weapon systems in late 1952, the
vehicles were found to be unsuitable for use
as BAT weapons carriers within an infantry
battalion. While the ONTOS vehicle provided
the necessary armor protection and fully-
tracked cross-country mobility, it had the
disadvantages of mechanical unreliability,
reduced accuracy, increased weight, inade¬
quate space for crew and ammunition, and
limited protection afforded by armor when
recoilless rifles were reloaded from outside
the vehicle (Ref. 4). As such, the M38A1 Jeep
Carrier was maintained as the interim carrier
for the Rifle, 106 mm, M40.
1-28
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AMCP 70&-238
1-6 OTHER LARGE CALIBER WEAPONS
(LARGER THAN 105 mm)
1-5.1 DEVELOPMENT OF 120 mm HAW
Standardization of the 106 mm, M40 BAT
System in 1954, satisfied the “interim”
requirements for an infantry heavy antitank
weapon (HAW) system. At this time,
Frankford Arsenal was authorized to initiate
R&D studies leading to the development of a
system to satisfy the following principal
“ultimate” military characteristics:
1. Destructive Capacity. Defeat 6 in. of
heavy armor at 64-deg obliquity, 90 percent
of the time.
2. Range and Accuracy. First round hit
probability of 0.75 at 2000 yd and 0.90 at
1000 yd.
3. System Weight. 200 lb desired.
4. Rate of Fire. 10 aimed rounds per
minute (not sustained). Go into action in 30
sec.
The first work specific to the ultimate
battalion antitank (U-BAT) requirements was
initiated in 1955 with caliber and lethability
studies. These activities were the basis for the
selection of a 120 mm caliber rifle for the
U-BAT weapon. The resulting design for the
120 mm rifle was patterned after the design
of the Rifle, 90 mm, T234 which was, in its
original conception, a design study for the
U-BAT application. Designated as the T246,
the 120 mm Rifle incorporated the segmented
nozzle and central orifice type breech design
used in the T234 Rifle. Development of the
T246 Rifle was suspended in December 1957.
In early 1959, the work begun under the
U-BAT program was reactivated under the
heavy antitank weapon (HAW) program. The
HAW program, however, was committed to
the development of a 2000-yd system which
would provide greater lethality, improved
range and accuracy, reduced system weight,
and improved simplicity and accuracy. The
first HAW design was designated as the XM89
and incorporated several features in its deugn.
The barrel of the XM89 Rifle incorporated
the strain compensation principle using steel
with a 160,000-psi yield strength. It was
expected that the entire HAW weapon would
use steel with a 200,000 psi yield strength,
but because of its limited availability at the
time, high cost and limited machinability, no
complete HAW rifle ever was made with this
material.
A second feature of the XM89 was the
incorporation of a variable control mount. By
use of a down range pointing joystick control,
the rifle could be moved in free traverse and
elevation for use against close and moving
target with a direct control ratio of 1:1. The
joystick control also provided variable control
ranging between a ratio of 9:1 and 36:1 for
laying the rifle on distant fixed targets. Other
unique features of the 120 mm, XM89 were
the use of a 15 mm spotting rifle and a trigger
firing mechanism from which both major and
spotting rifles could be fired with the same
trigger (Ref. 14).
During the second quarter of calendar year
1961, the development of the XM89 system
was divided into two directions. The first
direction being development with a frangible
cartridge case and the second, development
with a steel cartridge case. Development of
the frangible cartridge case system, designated
as the XM89E1, proceeded through initial
firing tests in a satisfactory manner. At that
point, the XM89E1 system was removed from
the development program. Development of
the perforated steel cartridge case system,
designated as the XM89E2 and later to be
known as the XM105 (shown in Fig. 1-12),
was slowed by minor strength problems and
mechanical difficulties, but finished with an
accelerated testing program in mid-1972.
While it was felt that, with small improve¬
ments, the XM105 system could be standard¬
ized, its excessive system weight (398 lb
1-30
Figure 1-12. 120 mm Rifle, XM105
mCP 706-238
instead of the desired 200 lb) and the
selection of the TOW missile to satisfy the
HAW requirements resulted in the termina¬
tion of the 120 mm HAW, XM105 program.
1-5.2 DAVY CROCKETT 120 mm, XM63
(XM28) AND 156 mm, XM64 (XM28)
An impending break-through in the design
of nuclear devices in the mid-1950's led to the
idea of a tactical nuclear weapon s> stem-per¬
haps capable of man-portability over limited
distances in difficult terrain. There was
growing confidence in the feasibility of
developing subkiloton devices lighter in
weight, structurally more resistant to accelera¬
tion stresses, and more efficient in the use of
critical nuclear materials than had been
available previously. Based on the estimates
available, a lightweight recoilless weapon
system concept was synthesized. It was
envisioned that such a system capability could
have important effects on the structure and
deployment of ground forces. The availability
of such huge elements of firepower in the
hands of foot troops capable of immediate
reaction to rich targets of opportunity could
provide an advantage of significant propor¬
tions.
The principal problems facing the designer
were as follows:
1. Design a breakdown system in which
each module can be carried over limited
distances by an individual soldier.
2. Accommodate the roughly 12-in. diam¬
eter warhead.
3. Limit maximum acceleration.
4. Insure high reliability and precision of
delivery. The problems that were encountered
resulted from the long range (4000 m for the
XM29 system) over which the spotting system
was to match the major caliber system.
5. Minimize system action time (i.e., the
time required to set up the weapon and
mount, insert the cartridge and projectile).
To meet the system module weight
limitation, high-strength structural materials
such as titanium alloys, glass fiber reinforced
plastic, and ultra-high strength steel were
considered despite their high costs. The
warhead size and acceleration problems were
accommodated by adopting the so-called
“spigot” configuration (Fig. 1-13). In this
scheme, the propulsion gases act upon a
“pusher” tube (spigot) throughout the ballis¬
tic stroke and the oversize warhead is
accelerated via this tube from a starting
position in front of the gun muzzle. This
avoids the structural problems of a large low
pressure gun tube and the interior ballistic
reproducibility problems associated with very
low operating pressures. High precision of
delivery was demanded by the need to insure
against injury to friendly troops as well as the
requirement to employ the extremely high
cost warhead effectively.
The projectile weight, size, velocity, and
acceleration limits along with the use of the
spigot configuration, allow the use of a
smooth bore barrel. Also, since the DAVY
CROCKETT systems are very low rate of fire
weapons, there is no need for any type of
breech mechanism. This leads to the muzzle
loading and simple nozzle end configurations
of the DAVY CROCKETT weapons. Another
unique feature of the DAVY CROCKETT
were the lightweight mounts. Weighing less
than 20 lb, the mounts had adjustable rear
lugs, fine and coarse elevation, and quick
collapse for stowage.
The operational requirements for a man-
portable system were met with two weapon
systems. The 120 mm, XM28 System as
shown in Fig. 1-14, was designed for a
maximum range of 2000 m. A second system,
155 mm, XM29, as shown in Fig. 1-15, was
developed concurrently to provide a 4000-m
system capable of field maneuver on a jeep or
other light vehicle. The XM29 System is
1-32
AMCP7M-238
W eap on
Figure 1-13. Recoilless Weapons—Conventional and Spigot Type
capable of firing from a jeep carrier or it can
be displaced by troops and fired from a
ground mount.
In the official press release of the DAVY
CROCKETT systems by the Army on 4 May
I960, the Secretary of the Army described
these systems as a development which
.. dwarfs in firepower anything we have
ever known in the immediate arta of the
battle line”. He stated that “DAVY CROC¬
KETT will significantly enhance the military
posture of the U.S. ground forces. With this
weapon, small combat units will have organic
atomic power that they will be able to take
with them to any trouble spot in the world in
a matter of hours. On the battlefield, the
small unit will have within its own ranks,
firepower that formerly could be obtained
only from heavy artillery". Among the
engineering advances achieved were:
1. First titanium gun in US Army
2. Hi-Low spotter cartridge-unprece¬
dented velocity uniformity
1-33
AMCP7084M
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Figure 1-15. DA VY CR0CKE1T XM29 Weapon System, 4000-m Range
3. PYROCORE primer—improved velocity
uniformity
4. Unique spigot delivery-minimum sys¬
tem weight and size
5. First use of D38 (reactor byproduct) in
spotter projectile
6. First multizone recoilless system.
The reason which made many of these
advances possible was that cost was not as
critical a factor, comparatively speaking, as it
was in most other recoilless rifle programs.
This was true because of the need for the
accurate delivery of the rather expensive
warhead. As such, the use of high cost
materials such as titanium was warranted in
the DAVY CROCKETT system and not in
others, even though experimental work on
titanium, Fiberglas, Fiberglas wound, and
glass-metal structures had given good results
in PAT, Super-PAT, and other recoilless
programs.
1-5.3 DEVELOPMENT OF 8-in. CANNON
(EIK)
In 1954-5S, the Office, Chief of Ordnance
directed Frankford Arsenal to study the
feasibility of producing an 8-in. recoilless
cannon (EIK) to replace the medium (8-in.)
howitzer as the general support in division
artillery and as the reinforcing weapon in
Corps artillery. Analyses and experimental
test firings with a scale model weapon, as
reported in Refs. IS and 16, showed that it
was technically feasible to obtain a weapon
system capable of air delivery for direct
support of infantry, which meets the follow¬
ing requirements:
1. Maximum Range: 30,000 yd desired
20,000 yd acceptable
2. Minimum Range: 3,000 yd desired
10,000 yd acceptable
3. Traverse: 360 deg desired
120 deg acceptable
1-35
AMCP 700-238
4. Elevation:
5. Carriage:
6. Ammunition:
0 to plus 65 deg
Unarmored and self-
propelled
HE, chemical, opti¬
mum fragmentation
(240-lb projectile)
Based on these findings, Frankford Arsenal
was directed to continue analytical studies
and proceed with experimental investigations.
Firings were to be performed in both reduced
and full scale test guns to verify the interior
ballistic predictions, to evaluate experimental¬
ly the effects of blast, and to investigate
methods of zoning. These tasks were to be
done for range requirements of 10,000 and
20,000 yd.
In addition, for the 20,000-yd range
weapon, studies were to be made of the
mechanical design of the cannon and design
concepts prepared illustrating methods of
incorporating the cannon into lightweight
towed and self-propelled weapon systems. A
designation of Cannon, 8-in. Howitzer, Re¬
coilless, T230E1 was assigned to the
10,000-yd weapon and Cannon, 8-in. Howit¬
zer, Recoilless, T230E2 assigned to the
20,000-yd weapon.
Experimental interior ballistic studies were
begun with a 75 mm scale model of the 8-in.
weapon, since previous studies had shown
that a small caliber test weapon could be
designed to have ballistic characteristics
similar to a large caliber prototype weapon.
The technique of “scale model” studies
resulted in considerable savings of both time
and money, since less effort was expended on
the experimental work performed. Empirical
data obtained during the scale model studies
were used in the design and construction of
the full scale, 8-in., 10,000-yd test Cannon,
T230E1.
ment, interior ballistic design, and general
performance. Based on performance results
from the scale model and full size test
weapons, the 8-in., 20,000-yd test cannon,
ignition studies, ballistic assessment, and long
range accuracy firings were performed. These
studies indicated that the 8-in. recoilless
cannon is lighter than conventional closed
breech weapons of equivalent range and fire
power, and is more accurate than rockets of
the same capability. Its corrected round-to-
round range dispersion was about 0.3 percent
of range. While it was found that the
problems associated with blast were no more
serious in the EIK than in comparable
rockets, it still was felt that the blast of an
EIK would have to be reduced.
While firings were being performed with
the EIK test cannons, preliminary weapon
system concepts were investigated by the
Pitman-Dunn Laboratories Group of Frank¬
ford Arsenal. These investigations included
breech design and concepts of vehicular
components and related ground handling
equipment for the EIK weapon system. Ref.
24 describes in detail the following weapon
concepts: swing breech, jackknife bieech,
reciprocating-pivoting breech, and spherical
chamber weapon. These weapon designs then
were studied for their adaptability to several
configurations of mounts and vehicle trans¬
ports. These studies indicated the following
weights for the indicated weapon systems
(Ref. 24):
Total System Weight, ton
8-in. Recoilless -
Weapon with 10,000-yd 20,000-yd
Towed mount 2 3
Austere self-pro¬
pelled carriage 4.5 6.5
A total of 25 rounds were fired in the 8-in.
T230E1 test weapon for charge establish¬
Armored self-pro¬
pelled carriage 8
12
1-36
AMCP 706-238
These weight figures along with accuracy and
fire power data indicated that in the
mid-1950’s, the 8-in. recoilless cannon repre¬
sented a reasonable replacement for existing
conventional artillery and rockets.
1-5.4 DEVELOPMENT OF SELF-EJECTING
BREECH
In June 1952, the Universal Winding
Company began work on the design, develop¬
ment, and fabrication of a scale model 105
mm recoilless rifle with a self-ejecting breech
mechanism. Following preliminary studies,
development proceeded along two separate
paths. The first path followed was the
continued development of Frankford Ar¬
senal’s “blowback” principle breech design.
This design used escaping propellant gases to
accomplish automatic cocking of the weapon
and ejection of the spent cartridge. The
blowback breech weapon as designed by
Universal Winding used a round with a
combustible case and a shouldered steel base.
In the base were a series of blowback orifices
(vent holes) through which the propellant
gases of the fired round were allowed to
escape. As the propellant gases escaped
through these orifices, they impinged on the
firing mechanism hammer, forcing it back to
its cocked position so that it is ready for the
next firing. The breech of the rifle is fitted
with a split ring that springs open to permit
chambering of the round. As the firing
mechanism is cocked by the escaping
propellant gases, a cam on the hammer
engages this split ring, forcing it open. The
timing of the hammer is such that when the
split ring is opened, the pressure in the
chamber has dropped to a level where it safely
blows out the case base to the rear (Ref. 19).
The second development path investigated
the designs of both electrically and mechan¬
ically fired drop-out breech designs. In the
drop-out breech, a set of latches or a breech
bar acts as a retainer for the cartridge base of
a combustible round. During the firing, the
chamber pressure forces the cartridge base
against the breech bar or latch detents which
held the base in place. When the chamber
pressure drops, the cartridge base drops
harmlessly through an opening in the breech
mechanism. At Universal Winding (Ref. 19), it
was felt that the blowback breech system
appeared to have more promise in recoilless
rifles of larger caliber where it would have a
weight and compactness advantage over
conventional or drop-out type breeches. In
lower caliber weapons, the blowback breech
would compare well with other breech types
in both weight and functioning, but would be
somewhat more complex in breech and firing
mechanism design.
1-6 RESEARCH PROGRAMS
1-6.1 INTRODUCTION
Except for a short period of time after
World War II, Frankford Arsenal was assigned
the responsibility for research and develop¬
ment of recoilless rifles. As a part of its effort
to broaden the industrial and engineering base
during recoilless rifle development, Frankford
Arsenal contracted several facilities to per¬
form research work in the refinement of
theoretical design concepts and the general
improvement of recoilless rifle technologies.
Notable among these facilities were A. D.
Little, Inc., Armour Research Foundation,
Firestone Tire and Rubber Company, Harvey
Aluminum, Midwest Research Institute,
United Shoe Machinery Corp., and Universal
Winding Company.
A description of all the recoilless rifle
research is beyond the scope of this
handbook, but the major research activities
performed by the previously mentioned
organizations are presented in the paragraphs
that follow. In these programs, direction and
technical specifications were given to the
contracted organization by Frankford Ar¬
senal, which then assumed a supervisory role
throughout the program.
AMC? 706-238
t42 MIDWEST RESEARCH INSTITUTE
1-&2.1 Gun T«mp*rs,tu r *
Among the activities performed at Midwest
Research Institute (MRI) were various heat
transfer studies for several recoilless rifle
programs. In connection with these studies,
MRI developed a special surface thermo¬
couple capable of measuring the temperature-
tune variations at the internal surfaces of a
recoilless rifle during firing. First used in
1952, to measure the internal surface
temperatures of a 57 mm, T15E13 Recoilless
Rifle, the thermocouple was improved by the
development and incorporation of special
high temperature insulation for the probe
assembly. With these improvements, the
thermocouple was capable of use in applica¬
tions involving surface temperatures up to
2000° F and was used extensively in tempera¬
ture-time studies of the internal surfaces of
the Rifle, 106 mm, M40 during the ballistic
cycle (Ref. 17).
One of the more extensive studies per¬
formed at MRI was conducted to determine
the permissible firing procedures for the Rifle,
105 mm, T170 (106 mm, M40). The
experimental program consisted of heating
the T170 Rifle under various firing conditions
in order to define the limitations that should
be imposed on the T170 Rifle. In all highly
Stressed weapons, these limitations arise
because the gun steel strength decreases with
increasing temperature. When fired under
adverse conditions, the rifle temperatures
approach the level at which the yield strength
decreases very rapidly and it is hazardous to
fire the weapon (Ref. 1).
Observed internal gas pressure data were
used in conjunction with the riP. stress
analysis and tensile properties of the weapon
material, to determine the maximum possible
rifle temperature. A thermodynamic analysis
then was made at selected points of the T170
Rifle to predict the rifle temperatures that
would result from certain rates of fire at
chosen ambient conditions. Heat transfer to
the rifle wail from the propellant gas was
determined experimentally by monitoring
twelve thermocouples attached to the rifle.
The experimental and theoretical works then
were compared to establish that the T170
Rifle was safe to fire at an initial burst of
twenty-one rounds at an ambient condition of
125°F without regard to rate. It also was
shown that the maximum safe rate of repeat
firing was 0.75 round per minute at the same
ambient condition. These firing procedures
were based upon a limiting rifle temperature
of 800°F.
1-622 Sheet Propellant Studies
During the development of the Frankfotd
Arsenal U-BAT Recoilless Rifle, MRI was
contracted to evaluate the feasibility of
caseless rounds for the U-BAT weapon.
Previously, considerable work had been
performed at MRI with the use of sheet
propellant charges in both fin- and spin-
stabilized projectiles. These studies had
indicated the feasibility of using sheet
propellants as a means of eliminating the
cartridge case. In order that no worthwhile
technique to eliminate the cartridge case
would be overlooked, sheet, granular, stick,
and slotted tubular propellants in various
combinations were tested in order to find a
satisfactory caseless round.
Since the U-BAT rifle and ammunition
designs were still in their formative stages
during the MRI investigations, if was decided
to perform the tests with a modified 105 mm,
M27 Rifle using fin-stabilized test slugs.
Embossed sheet propellant was used in disk
and scroll forms with different combinations
of each placed at various positions along the
projectile boom. Experimental firings indi¬
cated that a ballistic efficiency of 6.54 was
attained with the sheet oropellant as com¬
pared to 6.26 for a granular propellant. Even
though the sheet propellant gave a higher
ballistic efficiency, it is not possible to say
that the sheet propellant web is the most
AMCC 706-238
efficient. It also was found that the sheet
propellant rounds indicated no perceptible
unhumed propellant ejection from the rifle,
whereas, in the granular propellant rounds,
approximately IS percent of the propellant
charge was ejected unbumed. The various
projectile designs and the manner in which
the propellant was positioned around the
projectile in order to form the caseless round
are described fully in Ref. 18.
1-&2J Gun Dynamics
Successive recoilless rifle and mount
designs were built to higher performance
standards, while at the same time, their
weight was undergoing considerable reduc¬
tion. In order to ensure that the mount
strength and weapon accuracy are unaffected
by the vibrations caused by the recoil forces,
it is necessary to determine the magnitude of
these forces. As part of its studies on the
Rifle, 10S mm, T170 (M40), MRI conducted
pendulum-supported firings to determine the
weapon recoil force history.
Recoil forces were obtained from accelera¬
tion histories of the pendulum supported
weapon as MRI had done previously for the
37, 57, and 75 mm rifles. The acceleration
histories are determined by means of an
accelerometer that has its output filtered to
determine the various modes of vibration. As
described in more detail in Ref. 18, the T170
Rifle gave satisfactory recoil force histories.
1-6.2.4 Ignition Studies
As ballistic requirements became more
rigorous for the more sophisticated recoilless
rifles, it was apparent that ignition system
performance was a vital factor in the ballistic
cycle. As part of its BAT activities. MRI was
authorized to study the ignition process and
establish suitable criteria for evaluating
recoilless rifle ignition systems. concluded
in Ref. 18, a good ignition syv ivi is one
which:
1. Ignites the propellant in any air temper¬
ature from - 65° to + 125°F without hang-
fires or misfires.
2. Ignites the propellant in such a manner
that the burning propellant meets all interior
ballistic requirements such as:
a. Consistent projectile muzzle velocities
b. Smoothness of pressure-time curves
c. Unifonnity of peak pressures
d. Acceptable rates of initial pressure rise
in all parts of the chamber
c. Consistent projectile ejection time
f. Minimum breech and muzzle flash and
smoke.
3. Meets the general requirements of:
a. Minimum costs of ammunition
b. Suitability to mass production without
undue safety hazard
c. Stability in storage over long periods of
time under the conditions as nrescribed for
the propellant
d. A minimum of corrosive and toxic
combustion products.
1-6.2.5 Flash Characteristics
Midwest Research Institute also investigat¬
ed the breech flash of recoilless rilles with
the goal of accomplishing mechanical dash
suppression for the 105 mm recoilless ride. As
described in detail in Ref. 20. the research
program consisted of firing tests in both
vented chambers and a prototype weapon:
aerodynamic tests conducted in a gas
dynamics facility; and a theoretical investiga¬
tion of the Hash mechanism. As far as the
main question of this program, it was found
that a mechanical suppressor with reasonable
dimensions could not '.>e designed for the 105
1-34
AMC? 706-238
i
j
I
I
nun rifle. However, during the course of the
research, the following conclusions were made
about the nature of nozzle flash and nozzle
flow (Ref. 20):
1. There are several mechanisms by which
a secondary flash may be triggered. Shock
ignition may occur, or there may be a
continuous reaction from the nozzle exits. A
further possibility is that ignition may be
caused by energy transfers in the boundary
layer. Depending on conditions, some or all of
these may be active.
2. The peak pressure in the breech is
associated with the triggering mechanism. In
general, shock ignition will occur for a lower
peak pressure, and a continuous reaction from
the nozzle exits will occur at a higher peak
pressure.
3. The flash mechanism is more complex
and more difficult to suppress for a large
nozzle (as in the 105 mm rifle) than for a
smaller one.
4. In multiple nozzle systems, there may
be a strong interaction of individual jets
which may trigger the flash.
5. For the smaller nozzles those suppressor
configurations that most effectively destroyed
the shock structure of the flow did produce
the most effective flash suppression.
6. For conical nozzles, the presence of
normal shocks depends mainly on the
divergence angle. Larger divergence angle-
increases the possibility of a normal shock.
7. For the unsteady flow, divergence
angles of close to 35 deg can be used without
flow separation for expansion ratios of up to
about 77:1.
1-6.3 ARMOUR RESEARCH FOUNDA
TION
1-6.3.1 Interior Ballistic Theory
As part of the development of the
Battalion Antitank Weapon, Armour Research
Foundation was assigned the task to develop,
in cooperation with the Frigidaire Division of
the General Motors Corporation, a 105 mm
front orifice recoilless rifle. In connection
with this task. Armour Research Foundation
developed the interior ballistic theory for the
rifle.
The ballistic cycle of a front orifice is
composed of three stages:
1. A conventional closed breech phase
during which the projectile initially seals the
nozzle entrance ports
2. A transition stage from closed breech to
recoilless phase (partial recoil compensation)
3. A recoilless phase in \ .ich the nozzle
entrance ports are uncovered completely.
As shown in Ref. 28, accurate solution of
the three-stage ballistic system would require
the application of numerical methods of
calculation. For ease of computation and
analysis, personnel at Armour Research
Foundation devised a method of putting the
ballistic equations in closed form. This was
accomplished by reducing the 3-stage system
to a 2-stage system consisting of (1) a closed
breech phase lasting until the projectile
uncovers a portion of the total port area equal
to one-half the throat area, and then (2) a
recoilless phase thereafter with the discharge
of propellant gases controlled by the nozzle
throat (equivalent to assuming an instanta¬
neous transition from closed breech to
1-40
AMCP 706*238
recoilless operation). The interior ballistic
equations developed for a 2-stage system also
have application to a recoilless rifle with a
nozzle start device, such as a blow-out disk
(Ref. 4).
During the analysis of the 2-stage system of
interior ballistic equations and their applica¬
tion to the 105 mm T135 Rifle, several
observations and discoveries were made (Ref.
4):
1. Since the propellant gases initially are
confined in the chamber until the nozzle
ports are uncovered, the chamber temperature
at the beginning of the second stage is
approximately equal to the isochoric flame
temperature of the propellant.
2. The gas temperature then decreases to
0.6-0.7 of the isochoric flame temperature at
“all burnt” depending upon “all burnt”
velocity.
3. Through numerical integration of the
energy balance and mass balance equations, it
was found that the propellant gas temperature
may be represented fairly accurately by
assuming the average of the square root of the
temperature to be linear with projectile
velocity.
By use of these findings to simplify the
interior ballistic equations and then compar¬
ing the solutions to the solutions obtained by
assu. ing a constant value of propellant gas
temperature, it was found both calculations
were in good agreement with the solutions
obtained from numerical methods. On the
basis of these results, the simpler form of the
interior ballistic equations, based on constant
gas temperature, was used to predict, quite
accurately, the performance of front orifice
recoilless rifles.
1-6.3.2 Propellants
In connection with the development of the
interior ballistic equations for recoilless rifles.
Armour Research Foundation examined the
application of the interior ballistic theory to
recoilless rifles firing inhibited propellants,
composite charges, and liquid propellants. In
order to indicate the feasibility of inhibiting
propellant grains to provide a progressive type
of burning, a series of firing and closed bomb
tests were performed with M10 and Ml
Propellants. These studies (Ref. 25) showed
the effects on burning characteristics caused
by different conditions of inhibiting and the
use of different types of solvents used to
carry the inhibitor into the grain.
Interior ballistic equations were developed
for recoilless rifles filing a composite chaige;
i.e., a charge consisting of a mixture of
propellant grains of the same composition,
but different web sizes and geometric shapes.
The use of an exact form function for the
composite charge complicates the solution of
the interior ballistic equations, and it was
found that in many applications that an
equivalent charge i , a single web size may be
substitute'
Because hydrazine-hydrazine nitrate-water
propellants exhibit a lower flame tempera¬
ture, higher impetus, and greater reduction in
flash as compared to solid propellants,
significant attention was focused on the use
of liquid propellants in recoilless rifles. As
part of the nozzle erosion program at Armour
Research Foundation, the experimental deter¬
mination of nozzle erosion due to the firing
of hydrazine-type liquid propellants was
undertaken. Investigations included the estab¬
lishment of ballistic parameters and the
effects of water and nitrate content on the
liquid propellant burning rate for both
hydrazine and a hydrazine-hydrazine nitrate-
water propellant.
1-6.3.3 Expendable Cartridge Case
As part of the development of the
Battalion Antitank Weapons, Armour Re¬
search Foundation continued its study of
combustible cartridge case materials. The
141
AMCP 706-238
\,1F
,
M
" ,jfv.
fr '
/ I
• " A-
i^'V,
majority of this investigation centered around
cellulose nitrate plastic combustible rubber,
acrylic plastic, or a paper-base phenolic
material. The conclusion of this investigation
(Ref. 25) was that certain of the expendably
cased rounds showed considerable promise,
particularly those with the cellulose nitrate
and paper-phenolic cases. The hard rubber
and acrylic materials were considered un¬
worthy of further investigations.
It was established fairly well that perforat¬
ing the expendable cartridge case has very
little effect on the fragment size, but
probably has deleterious effects upon the
interior ballistic performance. Scoring, or
other methods of setting up stress concentra¬
tions in the outer surface of the case, proved
to be inadequate and of little use. The best
results were achieved with heavier wall,
paper-phenolic, solid frangible cases.
The cellulose nitrate cases under study
were of a convolute structure, formed by
rolling a cellulose nitrate sheet to a case
diameter, with cement between the con¬
voluted sheets to hold the roll together. As
such, the variables for an individual case
construction were the sheet thickness and the
amount of area to which cement was applied.
Ballistic test data indicate that higher muzzle
velocities are achieved with cases having a
greater cemented area, due to the strengthen¬
ing of the case, and thus, better ignition,
during the initial burning stages. Also, ballistic
and piezometric efficiencies were higher when
using a thicker sheet of cellulose nitrate.
1 -6.3.4 Nozzle Studies
Armour Research Founda.ion, as part of
the 105 mm Battalion Antitank Weapon
development, conducted extensive investiga¬
tions into nozzle performance and nozzle
erosion. These investigations included both
analytical studies and experimental observa¬
tions of the phenomena associated with
propellant gas flow through a recoilless rifle
nozzle and are described fully in Ref. 29.
One of the experimental nozzle studies was
the qualitative verification of the nozzle flash
theory of re-ignition. This theory supposes
that the gases at the nozzle exit are cooled by
rapid expansion, thus halting radiation. A
subsequent recompression, resulting from
oblique interrupting shocks and Mach rein¬
forcement, then reheats the gases and causes
further radiation. By high-speed motion
pictures of the nozzle flash from 105 mm and
2.75-in. recoilless rifles, it was shown that
nozzle flash phenomena do perform according
to the re-ignition theory.
Since the nozzle characteristics are an
important consideration in the design of a
recoilless rifle, the variation of nozzle thrust
with the nozzle characteristics was studied in
depth by the Armour Research Foundation.
The nozzle characteristics examined were
expansion angle, expansion ratio, throat area,
and approach area. The significant results
from these experimental studies (Ref. 25) are:
1. Expansion angles less than 45 deg give
essentially the same thrust, while with angles
greater than 45 deg, there is a significant
decrease in thrust.
2. Thrust unbalance calculated from
steady-state isentropic flow theory is in good
agreement with experimental thrust data.
3. "file percentage change in thrust unbal¬
ance is approximately 0.8 of the percentage
change in throat area.
4. The 57 mm Rifle, Ml8 was fired with
various internal chamber configurations and
nozzle approach areas to study their effect on
recoilless rifle operation. It was found that, in
general, as the rearward taper of the chamber
contours is changed from positive to negative
(positive taper indicating a larger diameter at
the rear of the chamber), the rifle becomes
unbalanced increasingly rearward if the
chamber volume and nozzle configurations
are kept constant.
i
‘1
I
3
I
1-42
!>
i
AMCP 706-238
Because a bore-size straight-pipe nozzle has
the advantage of simplicity and ease of
fabrication over a conventional converging-
diverging type of nozzle, both the interior-
and exterior-mounted straight pipe nozzles
were investigated by Armour Research
Foundation for possible use in recoilless rifles.
On the basis of the experimental data, a
bore-size perforated-pipe would give negligible
unbalance while exhibiting reasonable ballistic
performance. However, the use of a bore-size
perforated-pipe nozzle has certain drawbacks.
As the result of higher solid propellant loss
through the nozzle, the ballistic efficiency of
a recoilless rifle is lowered by the use of a
bore-size nozzle. In addition, the lateral gas
spray through the perforations in the pipe
causes a hazard to personnel in the immediate
region normal to the rifle axis at the breech
end of the weapon.
Other studies conducted jt Armour Re¬
search Foundation included nozzle erosion
studies with M10, T18, and T25 Propellants
and hydrazine and hydrazine-hydrazine ni-
tra*e-water liquid oropellants. Besides deter¬
mining the erosion rates caused by these
propellants at different firing rates, these
nozzle studies included the investigation of
recoil compensating devices for nozzle ero¬
sion. Various recoil compensating devices
were tested in 75 mm scale-model and 106
mm Rifle, T170E1 firings. The results of
these studies are described in detail in Ref.
25.
1-6.3.5 Stress Analysis
Because of the everpresent emphasis on
wei^t resolution and the incidence of case
rupture and bulging in some recoilless rifles.
Armour Research Foundation, as a part of its
general investigation of cartridge cases,
conducted stress analysis of perforated metal
cases. As a first step of the stress analysis, it
was assumed that a pressure differential exists
between the inside of the cartridge case and
the chamber. Furthermore, at some time
during the firing cycle, the excess internal
pressure becomes large enough to cause a case
failure. For purposes of the analysis, the
pressure differential was regarded as an
equivalent internal pressure resulting from
propellant burning or the mechanical com¬
pression of the propellant grains caused by
bursting of the igniter.
Considering the previous remarks, the
perforated cartridge case design problem
consisted of first analyzing the static problem
of a thin perforated cylindrical shell subjected
to uniform pressure with or without end
constraints. The overall problem was
approached from three directions, namely
(Ref. 4):
1. Yield stress calculations for a perforated
cylindrical shell subjected only to internal
pressure
2. Elastic stress and displacement calcula¬
tions for the cylindrical shell subjected only
to internal pressure
3. Bending stress cak intions for the
perforated cylindrical shell with internal
pressrre and built-in ends.
lue yield stress calculations are elementary
and useful as a guide to design limits, but give
no information about the effects of bending.
In analyzing problems (2) and (3), an
approximate method was adopted from which
useful information could be obtained. This
method proceeds along the lines of elemen¬
tary strength of materials, replacing the
perforated shell by a nonperforated one
having equivalent elastic constants that are
different for dilation and bending. This
approximate method is outlined fully in par.
11-17.5. In application to the 57 mm
perforated cartridge case, it was found that
the approximate method gave slightly overde¬
signed results, however, it was believed that
the results obtained by this method were still
useful, especially if properly correlated with
experimental observations (Ref. 4).
AMCP 706-238
Other stress analysis investigations conduc¬
ted by Armour Research Foundation were
experimental and theoretical analyses of the
106 mm Rifle, T170E1 and 75 mm Rifle,
M20. Operational stress levels and margins of
safety were determined for these operations
as described in Ref. 25.
1-6.4 FIRESTONE TIRE AND RUBBER
COMPANY
1-6.4.1 Aerodynamics
As part of its projectile development for
the 105 mm BAT weapon system, Firestone
performed aerodynamic studies on the T138
slow-spin Projectile (MOBY DICK) prior to
undergoing any extensive test firing program
(Ref. 21). Through the use of wind tunnel
tests at Aberdeen Proving Ground, Firestone
was able to define various weaknesses in the
T138 Projectile design so that the necessary
corrective actions and appropriate types of
test firings could be made. In the develop¬
ment of the T119 Projectile, in-flight
photography and extensive wind tunnel
experiments formed the basis for choosing a
fin-sweepback angle of 65 deg. Since the
T171 MOBY DICK-type projectile appeared
to be very promising, several aerodynamic
studies of various tail and nose configurations
of the T171 Projectile were performed. These
studies indicated that a 6-flnned tail with a
tee nose provided better aerodynamic stabili¬
ty than either egg-cup tailed or finned egg-cup
tailed projectiles with smooth noses.
1 -6.4.2 Fuze Studies
During initial design studies of the HEAT
round being developed by Firestone for the
BAT weapon system, it was concluded that
the fuzes used in the HEAT rounds for the
Rifle, 105 mm, M27 were not sufficiently
quick-acting. A quick-acting fuze is required
for the HEAT round because of the sensitivity
of shaped charges to standoff. As a result.
Firestone investigated the performance of
various types of fuzes in the HEAT rounds it
was developing. Among the different types of
fuzes investigated were the magneto fuze
developed by the Stewart Warner Corpora¬
tion, the push-button method or electric fuze
developed by the National Bureau of
Standards, the spit-back fuze extensively
studied by the Ballistic Research Laborato¬
ries, electronic controlled fuzes, and inertia
fuzes. The fuze eventually decided upon for
the standardized HEAT round for the BAT
weapon system was of the single action.
1-6.5 UNIVERSAL WINDING COMPANY
In June 1952, the Universal Winding
Company of Providence, Rhode Island was
given a contract to design, develop, and
fabricate a scale model of a 105 mm recoilless
rifle with a self-ejecting breech mechanism as
previously described in par. 1-5.4. Carried on
concurrently with this program were a
number of investigations of recoilless weapon
systems that were adaptable or applicable to
the self-ejecting breech design. As described in
Ref. 19, these investigations included central
nozzle designs of which two syslems-the
replacing nozzle system and the gas balanced
system—were thought to be the most
promising. The investigation into the central
nozzle concept led to the study of side-firing
mechanisms. One type of firing mechanism
studied was a circumferential primer that
extended around the base of the round.
Under this general program, Universal
Winding also designed and manufactured test
fixtures for lining 57 mm and 105 mm
recoilless rifle cartridge cases. A final task of
the program called for an investigation into
the design of a semiautomatic recoilless rifle.
Development of a side-loading, magazine-fed,
blow-back operated, repeating recoilless rifle
was carried through 50 percent completion of
a test model when work on this particular
weapon was discontinued.
As a part of the development of large
caliber rccoilless rifles, Universal Winding
studied the possible alternatives to the
1-44
AMCP 706-238
jackknife” breech system proposed by
Frankford Arsenal. Based on one of the
suggested breech designs, Universal Winding
prepared and submitted a proposal for a
complete large caliber rifle system. No further
efforts were performed in this area because
work on the large caliber recoilless rifles was
terminated shortly thereafter.
1-8.6 A. D. LITTLE, INC.
Having compiled an extensive bibliography
on recoilless rifle development and shaped
charge ammunition, A. D. Little, Inc. was
used extensively as a resource for historical
and background information as well as in
advisory capacities during many of the
recoilless rifle development programs. While
A. D. Little performed some work in
investigating possible solutions to the BAT
requirements, a great portion of its recoilless
rifle work was performed during its develop¬
ment of the lightweight Rifle, 90 mm, T149
for use as a platoon antitank (PAT) weapon.
During the development of the T149 Rifle,
considerable amounts of research were per¬
formed in the areas of central nozzle design,
rifle chamber contours, cartridge case and
liner designs, rocket-assisted and supersonic-
launched projectiles, and flash suppression.
Some of the specific outcomes of this work
were the development of a unique cam-ring
breech design, as described in par. 10-22 for
the T149 Rifle; the discovery that central
nozzles with divergence angles in excess of 40
deg could be used with substantial reduction
in nozzle length and consequently, nozzle
weight; and the establishment of the need for
still a better cartridge liner material than
nitrocellulose and polyethylene-coated Kraft
paper that had been two of more widely used
liner materials.
1-6.7 HARVEY ALUMINUM (HARVEY
MACHINE COMPANY)
As early as October 1951, the Harvey
Machine Company was performing compre¬
hensive studies on the overall problem of an
automatic recoilless rifle. Initial studies of the
57 mm rifle indicated that the advantages of
semiautomatic operation would be more
apparent in a large caliber weapon. According¬
ly, the 105 mm recoilless rifle was selected for
further development by several organizations
in 1952. Ref. 22 serves as an extensive record
of the ordnance experience and information
obtained by Harvey Machine Company in the
development of its own semiautomatic rifle.
Other research performed by the Harvey
Machine Company included the investigation
of lightweight alloys for application to large
caliber recoilless rifles and mounts, and the
development of an inexpensive, single-shot,
throw-away minor caliber spotting device.
Ballistically similar to a cal .50 spotting rifle
and ammunition, this spotting device consis¬
ted of an integral barrel and chamber
combination of very light weight material
with provisions for attachment to the major
caliber rifle by a clip-on device. The principal
achievements made in conjunction with this
task were the perfection of tooling and press
forming techniques for fabricating the barrel,
chamber, and rifling from a single aluminum
alloy bar slug; thus eliminating all the
customary machining performed on rifled
barrels and cartridge cases. These techniques
for low cost mass production of precision
press-formed riflings in high strength alumi¬
num alloys were considered to be applicable
to larger or smaller caliber weapons.
The last major work performed by the then
named Harvey Aluminum Company for the
recoilless rifle programs was performed in
1958-1962 in connection with the develop¬
ment of the DAVY CROCKETT weapon
system. Some of the major achievements
resulting from research activities on this
program (Ref. 23) were the demonstration
that:
1. Titanium was suitable as a recoilless rifle
material.
145
AMCP 706-238
2. Available coatings for titanium nozzles
were not satisfactory in erosion resistance.
3. Fiberglas, although a very capable
material, did not offer sufficient promise over
competitive materials to warrant the extensive
development effort that would have been
required to prove its suitability in the DAVY
CROCKETT application.
1-6.8 CARDE
The Canadian Armament Research and
Development Establishment (CARDE) was
responsible for several significant contribu¬
tions to recoilless rifle program. During the
BAT program, CARDE was responsible for
developing the process for embossing sheet
propellant. Sheet propellant is embossed to
provide the necessary space for gas flow
between adjacent layers of the sheet propel¬
lant. Along with its work in the area of sheet
propellants, CARDE developed a new type of
primer. The CARDE type, high pressure,
controlied-venting, hot gas primer employing
sheet propellant showed much promise. As a
result, similar CARDE type primers were
developed by other organizations for use in
the BAT weapon system. Other CARDE
activities centered around various analyses of
existing recoilless weapons and feasibility
studies of a medium antitank recoilless rifle.
1-6.9 FRANKLIN INSTITUTE
The Franklin Institute Laboratories for
Research and Development made two major
contributions to the recoilless weapon pro¬
gram. The first contribution was made in
1948-49 and consisted essentially of collect¬
ing all available literature associated with
recoilless weapons, reviewing the literature,
and compiling selected material in several
volumes of which Refs. 1 and 3 are a part.
The material was compiled into six volumes.
Volumes I, II, and III provide a history of
development and the basic principles of
recoilless weapons. Volumes IV, V, and VI of
the series on recoilless weapons contain
descriptive material on recoilless weapons that
were developed or were under development at
the time of writing in the United States and
abroad. This material was in the form of
reports and data which described recoilless
weapons from a mechanical and a ballistic
standpoint.
The second major contribution by Frankiin
Institute was the preparation of quarterly
progress reports on rccoilless rifle systems,
ammunition, and related items that were
being developed at Frankford Arsenal or
under the technical supervision of Frankford
Arsenal. The purposes of these reports were
fourfold:
1. To review local progress and effect
coordination as required
2. To serve as a repository for pertinent
classified data
3. To permit the transmittal of many
classified items in composite form
4. To maintain a backlog of data on these
projects so that the accumulation of these
progress reports would facilitate the prepara¬
tion of the final development reports.
1-46
AMCP 706-238
?
REFERENCES
1. Recoilless Weapons, Volume I, The Re¬
coilless Principle, A Symposium, Con¬
tract No.’s W-36-034-ORD-7652 and
-7708, Franklin Institute, May 15, 1948.
2. Rene R. Studler and W. J. Kroeger,
Battalion Antitank Recoilless Rifles
System, Report No. R1273, Pitman-
Dunn Laboratories, Frankford Arsenal,
July 1953.
3. Recoilless Weapons, Volume IV, Descrip¬
tion of Weapons, Contract No.’s
W-36-034-ORD-7652 and-7708, Franklin
Institute May 15, 1949.
4. Symposium on Recent Progress of
Recoilless Rifles and Ammunition, Held
at Midwest Research Institute, Sponsored
by Department of Army, 11-13 January
1954.
5. W. P. Leeper, Ammunition for 2.75-inch
Recoilless Rifle for Aircraft Installation,
Report No. 1376, Frankford Arsenal,
June 1957.
6. C. Walton Musser, Edward R. Barber,
George Schechter and P. J. Wilds, Strain
Compensated Barrels, Report No.
R-1008, Pitman-Dunn Laboratories,
Frankford Arsenal, May 1951.
7. J. E. Copeland and P. J. Wilds,
Development and Manufacture of Car¬
tridge T115E2 for the 57 mm Recoilless
Rifle T66E2, Report No. R-1007, Pit-
man-Dunn Laboratories, Frankford Ar¬
senal, May 1951.
8. G. S. Bluford, F. W. Dietch and F. J.
Shinaly, Rifle 57 mm T66E2, Report No.
R-1096, Pitman-Dunn Laboratories,
Frankford Arsenal, August 1952.
9. G. S. Bluford, Rifle 57 mm T66 and
T66EI, Report No. R-1011, Pitman-
Dunn Laboratories, Frankford Arsenal,
May 1951.
10. Development of the 90 mm Rifle TI49,
Interim Technical Report, Contract No.
DA-19-020-ORD-40, Arthur D. Little,
Inc., June 1, 1955.
11. Recoilless Rifle Systems, Ammunition
and Related Items, Status Report No. 1,
Vol. IV, Report No. R-1316, Frankford
Arsenal, 1 January through 31 March
1956.
12. Recoilless Rifle Systems, Ammunition
and Related Items, Status Report No. 3,
Vol. Ill, Report No. R-1282, Frankford
Arsenal, 1 July through 30 September
1955.
13. Recoilless Rifle Systems, Ammunition
and Related Items, Status Report No. 2,
Vol. II, Report No. R-1238, Frankford
Arsenal, 1 April through 30 June 1954.
14. Development of 120 mm Recoilless
Heavy Antitank Weapon System (HAW),
Final Report, Technical Memorandum
M64, Frankford Arsenal, 1 April 1959
through 30 June 1962.
15. A. E. Clark, et al., Feasibility Study of a
Large Caliber Recoilless Rifle, Report
No. R-1247A, Pitman-Dunn Laborato¬
ries, Frankford Arsenal, March 1955.
16. G. Schecter and L. W. Insetta, Large
Caliber Recoilless Cannon (EIK), Report
No. R-1345, Pitman-Dunn Laboratories,
Frankford Arsenal, March 1956.
17. Heating of the Tl 70 Rifle Under Various
Firing Conditions, Phase Report No. 1,
Contract No. DA-23-072-ORD-637, Mid¬
west Research Institute, December 1953.
18. Investigation in Connection with Battal¬
ion Antitank Recoilless Rifles. Final
Report, Contract No. DA-23-
072-ORD-900, Midwest Research Insti¬
tute, November 1955.
1-47
AMP* 706-238
19. Research and Development of Recoilless
Weapons, Final Report, Contract No.
DA-19-020-ORD-1848, Universal Wind-
... ing Co., 28 February 1957.
20. Research on Basic Studies of Flash
Characteristics of Recoilless Weapons,
Final Report, Contract No. DA-23-
072-ORD-762, Midwest Research Insti¬
tute, 30 September 1955.
21. 105 mm Battalion Antitank Project, First
Progress Report, Contract No. DA-33-
019-ORD-33, Firestone Tire & Rubber
Company, August 1950.
22. 106 mm Semi-Automatic Recoilless Ri¬
fle, Summary Report No. HMC-1009,
Contract No. DA1-04^95-507-ORD-(P>
14, Harvey Machine Company, 18 June
1957.
23. Study Re Battle Group Systems, Final
Summary Report No. HA-1862, Contract
No. DA-04-495-507-ORD-1283, Harvey
■Engineering Laboratories, Harvey Alumi¬
num, 15 June 1962.
24. R. T. Fillman and D. E. Walters, Large
Caliber Recoilless Cannon (EIK), Report
No. R-1533, Pitman-Dunn Laboratories,
Frankford Arsenal, March 1960.
25. Battalion Antitank Weapons, Final Re¬
port, Contract No. DA-11-022-ORD-
1157, Armour Research Foundation, 15
December 1955.
26. Recoilless Rifle Technical Information
Index (1944-1958), Publication Bulletin
PB8, Frankford Arsenal, September
1959.
27. Recoilless Rifle Technical Information
Index (1958-1962), Publication Bulletin
PB8, Supplement 11, Frankford Arsenal,
1962.
28. Samuel Levin and R. G. Wilson, Jr.,
Development of 105 mm Battalion
Antitank Weapons and Interim Ballistics
for the Design of Recoilless Rifles,
Summary Report, Vol. II, Front Orifice,
Armour Research Foundation, Project
No. L034, 1 July 1954.
29. Ramon L. Olson and A. D. Kafadar,
Nozzle Erosion Studies, Final Report,
Armour Research Foundation, Project
90-812L, December 20,1951.
CHAPTER 2
AMCP 706-238
SYSTEM DESIGN AND INTEGRATION
2-0 LIST OF SYMBOLS
A = bore area, in?
A t - nozzle throat area, in?
B - effective burning rate constant,
in.-fsec-psiT 1
C d = discharge coefficient of nozzle,
dimensionless
C ( = initial propellant charge, lb
C t = total weight of unbumed propel¬
lant ejected, lb
C 3 - propellant charge burned in rifle,
C 2 *C ( - c t , lb
c = specific heat at constant pressure
(ft-lbHlb^Rr 1
c v = specific heat at constant volume,
(fMbXlb-°Rr‘
E = energy, ft-lb
e = 2.7182818... base of natural
logarithms
F = propellant impetus, (ft-lb Mb' 1
/ = safety factory,/® o y /a ( > 1,
dimensionless
JIK V m ) = \p' m kF/(A/A t ), dimensionless
g - acceleration due to gravity,
ft-sec -2
I - impulse, lb-sec
K = nozzle coefficient, sec -1
L m - travel of projectile to muzzle, in.
L p * travel of projectile when peak
chamber pressure occurs, in.
M - weight of projectile, lb
m - mass of projectile, slug
m' - effective mass of projectile
N 0 - weight of propellant burnt at
projectile start, lb
P b - space mean pressure at time
charge is all-burnt, psi
P c * chamber pressure, psi
P e ® exit pressure at nozzle, psi
P„ ~ space mean pressure when pro¬
jectile is at muzzle, psi
I
r . r
^ i
b
".TTT*
AMCP 708-238
1
1-
- maximum pressure, psi
= pressure at nozzle throat, psi
Ri
= gun tube radius, in.
-
/
= CjifiA)
K ft “ velocity of projectile at all-
burnt, fps
V m - muzzle velocity of projectile, fps
y =jvelocity of projectile at peak
* chamber pressure, fps
*9%
a = C,/(pv c ), dimensionless
y = ratio of specific heats,
y - c p /c y , dimensionless
6 = (e - a)/(1 - a), dimensionless
A a = initial solid propellant loading
density, 21.1 C t /v c , g-cm -3
Total Gun Volume
e - expansion ratio,--,
, Chamber Volume
dimensionless
!
| A!
&
Ift
X
I
t
1
\
I
v. = chamber volume of rifle, in?
W m = density of metal, lb-ff 3
W a = initial web thickness of propel¬
lant grains, in.
W f =* weight of bate rifle, lb
W g - density of steel, lb
w m ~ wa ^ thickness corresponding to
pressure at muzzle, in.
w p - wall thickness corresponding to
peak pressure, in.
x M = effective length of rifle such that
Ax m is the total volume of rifle,
(i.e., Ax m ~ v c + AL m ) ,in.
x 0 - effective length of chamber such
that Ax o - v c ,in.
x p =x 0 +L p ,in.
X = kA t Wj(C 2 B\ dimensionless
p = density of propellant, lb-in“ 3
p' - density of gun material, Ib-inT 3
a - allowable tensile strength of the
material, psi
Oy - stress on rifle tube in y-direction,
psi
a t = tangential stress on rifle tube, psi
\l/' b = value of \li' for V - V b , (fps) -1
4/' m = value of for V=V m , (fps)'*
\p' o = value of i//' for V = 0, (fps) -1
2-2
[
!' ; j, $L_ ., . s~ — l- vr■■ ■..
-'•.UP'*' - ■ ; tj n
AMCP70C-23
SECTION I
INTRODUCTION
2-1 SCOPE
This chapter describes the logic, technique,
and philosophy of integrating a new recoilless
weapon system—of “putting it all together”
so that the end product serves the customers
needs and improves the overall defense
posture. It treats, more quantitatively and in
more explicit detail than does Chapter 1, the
definitions of subsystems and components
and the design trade-off opportunities avail¬
able throughout the engineering interval.
Stress is laid on the criticality of the early
trade-off analyses when this is possible. The
selections among basic altematives-such as
warhead type, projectile stabilization mode,
combustible versus frangible versus metal
cartridge case, expanded versus bore-size
chamber, spigot versus bore-size projectile and
the like-become more and more irreversible
as the investments of dollars and time grow.
Escape from these is often costly in both
material terms and professional pain. Some¬
times, of course, they are inescapable and
either the project is terminated or the defects
in the end product haunt you. Insofar as they
are instructive, some specific case histories,
failures as well as successes, are outlined.
Emphasis is placed on the advantages of the
integrated system approach. The continuing
tendencies toward specialization in modem
technology can lead to compartmentalization
and tends to yield a system with incompatible
interfaces. For example, a seemingly simple
bracket for attaching the telescope sight to
the gun tube will not be compatible with the
gun tube, if the sight bracket designer did not
consider the dynamic elastic behavior of the
tube under ballistic stress. Such pitfalls are
illuminated in this chapter and means to avoid
them (that have succeeded in the past) are
described.
2-2 DEFINITION OF TERMS
To define is (by definition) one of the most
arbitrary intellectual activities of man. Never¬
theless, the “labels” by which we designate
things and the meanings of these labels are
indispensable tools for our efficient function¬
ing, especially where engineering endeavors
are concerned.
Following is a list of specialized terms used
frequently in recoilless weapon system design.
These definitions are included here since they
are not found in the volume of Ordnance
Technical Terminology (Ref. 1). All the other
terminology not defined herein is fully
described in Ref. 1 and is not repeated in this
handbook. The asterisked (*) definitions
represent an updating of the term as it applies
to recoilless weapons rather than the defini¬
tion given in Ref. 1.
1. Blowout (or rupture) disc: Deliberate
obstruction to gas flow to nozzle, designed to
be removed by internal pressure of a
predetermined level
2. Bore area: Cross-sectional area of gun
tube (within lands)
3. Case liner: The membrane covering case
perforations to retain granular propellant and
exclude moisture
4. Chamber volume: Volume available for
propellant gases between projectile in rest
position and plane of nozzle throat
2-3
AMCP7W-233
5. Gun expansion ratio: Ratio of total gun
volume available for propellant gases at
instant of projectile exit to the chamber
volume
6. Loading density*: Ratio of charge
weight to chamber volume
7. Nozzle*: Duct through which a portion
of the propellant gases are directed rearward
to balance the momentum of the forward
moving projectile, thus creating a zero recoil
condition in the weapon
8. Nozzle entrance area: Cross-sectional
area in upstream portion of nozzle where
convergence begins
9. Nozzle erosion: Loss of material from
nozzle interface as a result of being exposed
to exhausting propellant gases
10. Nozzle exit (or mouth) area: Cross-
sectional area at downstream extremity of
nozzle.
11. Nozzle expansion angle: Included
half-angle of nozzle expansion cone
12. Nozzle expansion ratio: Ratio of exit
to throat areas
13. Nozzle throat area: Smallest cross-
sectional area of nozzle
14. Perforated case: Metal cartridge case
similar in general form to conventional cases
but with sidewall multiperforated for propel¬
lant gas emission
15. Piezometric efficiency: Ratio of aver¬
age to peak pressure (Ref. 2, p. 2-29)
16. Projectile travel: Distance from rear of
obturator (rotating band), when seated in
forcing cone, to end of tube
17. Propellant constants: Chemical com¬
position and physical dimensions of solid
granular propellant
18. Propellant force or impetus: Thermo¬
chemical energy available per unit weight of
propellant
2-3 GENERAL PRINCIPLES OF OPERA¬
TION
A recoilless (open-breech) gun, like a
closed-oreech gun, is essentially a single
cylinder heat engine that “loses its piston"
(projectile) with each cycle. However, unlike
the traditional gun-which transmits the recoil
to the earth through a system of slides,
hydraulic-mechanical devices, and supporting
structures—the recoilless gun counterbalances
the recoil force with the thrust of a “rocket
motor". This “rocket motor" shares the gases
generated in the gun chamber; some of the
gases propel the projectile and some are
discharged through the recoil balancing
nozzle. About 3-4 times more propellant is
needed in the recoilless system to do this, as
compared to closed breech guns. Also, the
“rocket motor” shares the gun structure of
the chamber, breech, and nozzle. Schematical¬
ly, one can visualize the recoilless system as
shown in Fig. 2-1.
If this concept a r . portrayed in Fig. 2-1
were reduced to practice, the following
difficulties and inefficiencies could be pre¬
dicted:
1. Simultaneous ignition of the two
propellant charges would be difficult to
assure, as would precise congruity of the
pressure versus time functions in the two
chambers. Large transient unbalanced axial
forces would result.
2. Pressure loads on the “fictious parti¬
tion” would be considerable both as a
function of ballistic time and physical
position. This structure and its supporting
chamber wall, consequently, would have to be
designed to withstand the maximum of such
transient loads, adding substantial weight to
the gun.
3. Dual ignition systems, dual propellant
2-4
Figure 2 - 1. Schematic Functional Diagram Showing a Gun Back-to-hack With a
Rocket Motor To Achieve RecoiUeaneu
charges, and separate loading mechanisms
would be required. All of these difficulties
and inefficiencies are eliminated by the basic
design that has been adopted -the basic design
shown schematically in Fig. 2-2 eliminates the
partition and shares the propellant gases and
structures for both the formation of the recoil
balancing jet and for pushing the projectile.
The application of the momentum balanc¬
ing principle has been made possible and
further refined through such developments as
the perforated cartridge case, kidney-shaped
nozzle, nozzle cant for spin compensation,
and others which are described more fully in
Port Three. Design, of this handbook. It is the
application of this principle that yields a
lighter weight system which does not penalize
accuracy, but has the disadvantages of higher
propellant weight, rearward blast with its
operating hazards, and intense visual and
auditory signatures.
The fundamental principles governing the
gas flow through the recoilless gun nozzle and
the formation of the jet are similar to those of
a rocket and are illustrated in Fig. 2-3. The
high pressure P ( of the gases generated in the
combustion chamber accelerates the projectile
by applying pressure P b to its base as in
conventional guns. Some of the gases move in
the opposite direction, converging through
the nozzle entrance and accelerating to local
sonic velocity at about half chamber pressure
P t in the nozzle throat. In the expansion
cone, the gases continue to accelerate into the
supersonic region and the pressure continues
to drop. At the nozzle exit, the gases transit
from the region of constrained expansion to
free expansion, provided the expansion angle
has prevented flow separation and the
expanded gases are still above ambient
pressure.
Since the gas is highly turbulent in the
2-S
AMOTOm
.f
:#
i
i
#-
"S»
Propellant Projectile
Chamber
Figure 2-2. Schematic Recoilless Gun
combustion chamber and the chamber pres¬
sure is a rapid transient (of the order of 10
msec), it would be erroneous to visualize the
flow conditions as laminar and steady-state.
Nevertheless, the steady-state laws describe
the phenomena adequately for gun design
purposes, and the more comprehensible
picture of steady-state laminar flow is useful
(and more comfortable), provided that one
realizes this is an idealization. The general
“rules” of basic nozzle design are given briefly
in Section III of this chapter and detailed
engineering design guidance is given in
Chapter 6, “Cancellation of Recoil”.
Beyond the nozzle exit is a large region of
free expansion and turbulent mixture of the
emitted products with ambient air. The gases
enter this region at high velocity (about 6000
fps); they contain large volumes of intermedi¬
ate products of combustion and significant
quantities of unbuml solid propellant; and
the area is criss-crossed with shock waves,
some of them very strong shocks in the
locations just downstream of the nozzle exit.
Secondary combustion occurs, producing the
characteristic large flash, blast, and smoke
phenomena. It also is noted that for a given
muzzle energy, the muzzle blast from a
recoilless gun is no more than that of a closed
breech gun. The danger zone created by the
gases exiting from the nozzle is conical in
shape with its apex at the nozzle. As shown in
Fig. 2-4, the danger zone for the 120 mm
HAW Weapon is approximately 130 ft deep
and 150 ft wide at its base (Ref. 3). For a
weapon as large as the 8-in. recoilless cannon,
the cone is approximately 400 ft deep and
500 ft wide at its base (Ref. 4).
2-6
Expansion Cone
Figure 2-3. Gas Flow in the Chamber an*. Nozzle
A - Rear Danger Area Due to Blast and Flying Particles
B - Area Considered Safe for Personnel Facing to Rear
Figure 2-4. Rear Blast Danger Area of Rifle. 120 mm, XM105
/UUCP 706-238
SECTION II
SYSTEM REQUIREMENTS
2-4 GENERAL
It is possible to outline the various input
requirements of the Tecoilless weapon system
in order to see how they relate to the basic
design output-weapon system weight. Fig.
2-5 is a block diagram showing these system
requirement relations. As seen at the top of
this diagram, the basic input requirement to
the weapon system is the kill probability for a
particular target and specified range.
As shown in Fig. 2-5, the kill probability
requirement, in turn, places certain require¬
ments on the hit probability and fire power of
the weapon being designed. As these require¬
ments ore traced further through the system,
it is found that all components of the rifle are
affected. The result of this interaction is a
system weight for a given terminal ballistic
requirement. The remaining paragraphs of
Section II more fully describe these require¬
ments and how the problem they create is
solved by determining the design parameters
that minimize weapon weight.
2-6 REQUIRED MUZZLE ENERGY
2-5.1 KILL PROBABILITY (See Chapter 7)
As stated in Chapter 7, single shot kill
probability is defined as the product of hit
probability and the conditional probability of
a kill given a hit. From the definition of target
vulnerable area, conditional kill probability
can be expressed as the ratio of the vulnerable
area to presented area. From Fig. 2-5, kill
probability is shown to be dependent on the
type of target and the type of gun-ammuni¬
tion-fire control combination used in attempt¬
ing to defeat the target.
2-5.2 HIT PROBABILITY (Sm Chapter 7)
Hit probability is defined as the probability
of a hit or hits on a target occurring out of a
given number of rounds fired at the target.
For a specified target and weapon system, the
hit probability then depends only on the
overall weapon dispersions. The principal
sources of these dispersions or firing errors are
range estimation, aiming, muzzle velocity
variation, system jump and cant, crosswind,
and the fire control equipment. Weapon
system design, production control, and
operator training attempt to minimize the
random errors contributed by the weapon
system and the gun crew. During weapon
system design, it is possible to minimize the
effects of the nonrandom errors. However,
this effort will not be made possible without
making some trade-off with the weapon
system weight.
One method of increasing the first round
hit probability is to increase the muzzle
velocity of the projectile. A high muzzle
velocity minimizes such errors as range
estimation and crosswind, but is achieved by
increasing the gun tube length or increasing
the chamber pressure-both of which result in
an increase in the weapon weight. A
sophisticated fire control system could also
increase the hit probability, but again, a
significant penalty is paid by the additional
weight to the weapon. In the last recoilless
weapon systems developed (BAT, MAW,
HAW, DAVY CROCKETT), it was found that
the use of a spotting rifle or spotting pistol
presented the most favorable compromise
between increased hit probability and in¬
creased weight.
Kill Probability
Weight
Figure 2-5. System Requirements
AMCP706-23S
Initial R»rgy of Projectile, ft-lb
Figure 2-6. Weight of Weapon vs Initial Energy of Projectiles for Recoilless Systems
2-5.3 VULNERABLE AREA (Sm Chapter 7)
The vulnerable aiea of a target is defined as
the product of the target presented area and
the conditional probability that a hit on this
presented area will be a kill. For a specific
type of warhead, achieving a higher condi¬
tional kill probability requires a larger caliber
warhead to accommodate a larger explosive
charge. As the projectile caliber largely
determines both the projectile and weapon
weight, an increase in caliber results in a
significant increase in the weapon weight.
2-6 WEAPON SYSTEM WEIGHT (Sm Chap¬
ters)
For a weapon system with a specified
round of ammunition, the system weight is
determined primarily by the required muzzle
energy. Increasing the projectile energy can be
achieved by either lengthening the gun tube
to increase projectile travel in the weapon or
increasing the chamber pressure of the
weapon. Both methods result in an increase in
the bare rifle weight. This increase in weight is
further compounded in the overall system
weight since it will be necessary to strengthen
the weapon mount in order to support the
heavier rifle. The effect on bare rifle weight as
caused by changes in the projectile energy and
momentum are shown in Figs. 2-6 and 2-7,
respectively.
The bare rifle weight is calculated from the
rifle dimensions and the internal pres¬
sure-projectile travel history as outlined in
par. 5-3 5 and is not a difficult task. However,
as described in Section HI, the more difficult
problem of the interior ballistician is to
determine the set of propellant and weapon
parameters which will minimize the system
weight.
2-11
AMCP7M-23S
SECTION III
DETERMINATION OF BALLISTIC PARAMETERS*
2-7 DETERMINE THROAT AREA
In Chapter 6, it is stated that for the open
breech weapon to be recoill e ss, a certain ratio
of bore-to-throat area is required for a given
nozzle expansion ratio. From a ballistic and
nozzle efficiency viewpoint, it is desirable to
use a large expansion ratio nozzle since this
permits the use of a small throat area, which
acts as a deterrent to the loss of solid
unbumed propellant. Secondly, the use of a
large expansion ratio results in a smaller
portion of the propellant charge being used to
balance the projectile momentum; i.e., a
smaller amount of the propellant gases,
expanded to a higher nozzle exit velocity,
achieves the same necessary balancing mo¬
mentum that would be attained by using
more of the propellant gas but expanded to a
lower velocity. As a result, it would be
possible to conserve a significant portion of
the propellant charge with a large expansion
ratio nozzle.
There is, however, a penalty that arises
from using a large expansion ratio nozzle and
it is in the form of additional weight to the
weapon. Compared to a low expansion ratio
nozzle, the lar*.e expansion ratio nozzle is
larger in actual space required and is
proportionately heavier, the gieaU the
expansion.
!n the appendix of Ref. 5, it is indicated
that nozzle expansion ratios of 1.79 to 3.S0
have been used in the various recoilless rifles
that have been designed and tested. The
specific nozzle expansion ratio to be used will
depend upon the type of nozzle employed in
the weapon and the compromises that are
made between efficiency and weight. For
example, in a central nozzle weapon, it is
possible to maintain a high expansion ratio,
*S#e Chapter 5.
with its significant weight decrease, by
increasing the divergence angle of the nozzle.
A loss in efficiency results, but the weight
savings from increasing the divergence angle
to as high as 45 deg (Ref. 6) can be
significant. Thus, the designer must decide
what compromise between weight and effi¬
ciency maximizes the weapon system effec¬
tiveness. In past recoiiiess rifle designs, nozzle
expansion ratios of 2.0-2.5 have been the
most widely used and seem to indicate that
they represent the best compromise between
efficiency and weight. For the nozzle
expansion ratio of approximately 2.0, it is
found that the bore area to nozzle throat area
ratio should be 1.45, thus determining the
nozzle throat area.
2-8 DETERMINE GUN AND PROPELLANT
REQUIREMENTS
The previous paragraphs have described
how the initial values of the bore and nozzle
throat area, projectile weight, and the muzzle
velocity are chosen. In Chapter 5, the system
of interior ballistic equations is shown not to
be readily solved until certain additional
variables are determined. These variables are
the peak chamber pressure, propellant charge
weight, and the propellant wvb. With a
specific choice of these quantities, it then is
possible to determine the chamber volume
and barrel length of the rifle.
The most desirable rifle is, of course, one
that is both light and short. Since a light rifle
corresponds to a low peak pressure while a
short rifle requires a high peak pressure, a
compromise between peak pressure and the
rifle size will have to be made. In order to
make the optimum compromise between peak
pressure and gun volume, it is necessary to
determine the relation between peak pressure
and gun volume. For a given peak pressure.
12,000
10,000
. ^ V-i
•H
. hr*
a
m
£
3
a
<a
*v.<.'
0)
£:
s
a
Xt
' man?,'
f : r : M.
O
jjj 6,000
4,000
2,000
Barrel Travel, in.
Figure 2-8. Pressure vs Trsvei 120 mm HAW Recoiiim Rifle
however, there is an infinite number of gun
volumes corresponding to different choices of
charge weight and web size. The designer's
problem then is to determine the minimum
gun volume for a given peak pressure.
Section V of Chapter S shows that it is
possible to integrate the system of interior
ballistics equations if an avenge gus tempera*
ture is u sed in place of the instantaneous gas
temperature. For a given set of ballistic
parameten, it is possible to use this method
of solving the interior ballistic equations and
determine the pressure-travel and velocity-
travel curves. As an example, Fig. 2-6 shows
the pressure-travel curve initially calculated
for the 120 mn» HAW weapon. Since the bore
area is specified, there is a required area under
the prescure-tnvel curve for which the work
done by the propellant gases in accelerating
the projectile is equal to the desired muzzle
energy. By varying the values of the gun and
propellant parameters, it would be possible to
2-14
AMCP70t2M
generate an infinite number' of differently
shaped pressure-travel curves for which thr
desired muzzle energy is attained. Since each
of these curves corresponds to a different set
of parameters, the task of evaluating one
system versus another would be extremely
difficult.
Since the weapon weight is primarily a
function of the gun volume and peak
pressure, it would be desirable to select a peak
pressure and then determine the ballistic
parameters that result in the minimum weight
gun. Section VI of Chapter 5 describes a
method that gives this desired result. For a
given peak pressure and value of the
dimensionless parameter, \ m kA,W a l(C 2 B), it
is shown how to use solutions of the
simplified interior ballistic equations of
Section V of Chapter 5 and be able to
calculate the weapon weight. By choosing
appropriate values of X, it is possible to
generate a family of curves showing the
weapon weight as a function of X for various
peak pressures. However, Section VI of
Chapter 5 indicates that there is only one
optimum value of X which satisfies the
minimum weight condition and outlines how
this value of X is determined.
For the optimum value of X, a curve of
weapon weight versus peak pressure is
obtained. The design that gives the lightest
weapon then is chosen-provided that the
peak pressure is not so high that it would
induce excessive erosion or blast and the
corresponding propellant loading density is
practical. The optimum value of X determines
the propellant charge and the other propellant
parameters. With this information, it is
possible to calculate the value of the
propellant loading density . For recoilless
rifles to be efficient, as described in Section II
of Chapter S, it is necessary that the value of
the loading density be about 0.6 g-cm* 3 •
Values above 0.6 tend to give highly peaked
pressure-travel curves resulting in low piezo¬
metric efficiency and exacting a penalty in
gun weight. Values below 0.6 tend to increase
the gun volume and weight.
It should be noted that the minimum
weight rifle solution does not spotify
completely the final parameters of the
recoilless weapon system. What it does is
specify a small range of peak pressures for the
optimum value of X which leads to the
minimum weight gun. Then, it is possible to
perform the calculations outlined in Section
V of Chapter S for the limited range of peak
pressures and the optimum value of X to
determine the complete set of weapon system
parameters. Through this procedure, the
designer has gone from evaluating an infinite
number of possible combinations of weapon
parameters, to evaluating the solutions ob¬
tained from 2 or 3 different peak pressures.
2-9 VERIFY CALCULATIONS WITH TEST
WEAPON
Once the specific gun and propellant
parameters have been determined, it has been
general practice to construct a full scale test
weapon in order to verify these theoretically
established values. The test weapon is
designed with the same ballistic characteristics
as the proposed weapon and is equipped with
a very simplified configuration of fire breech
design. In the design of very large caliber
recoilless weapon, the 8 in. recoilless cannon
beinR a prime example, a possible intermedi¬
ate step would be the construction of a scale
model test weapon in which preliminary test
firings are performed. The use of the scale
model test weapon greatly eases the transition
from theory to full scale weapon testing while
achieving significant savings of both time and
money.
Experimental firings of the test weapon are
conducted with projectiles cut from cylindri¬
cal steel slugs. The propellant charge is
contained in a cardboard tube, which serves as
the cartridge case, and is positioned in the
chamber behind the projectile. Ignition of the
round is performed through the use of an
2-15
electrical type squib or detonator.
Initial firings are performed to establish the
composition and quantity of the propellant
charge required to attain the desired peak
pressure and muzzle *• <yuty. The exact value
of the nozzle thro , and entrance and exit
areas are establish A at this time so that the
specified recoil cancellation is attained. Once
the charge establishment, interior ballistic
design, and general performance requirements
have been met, the test weapon is used for
ignition studies, ballistic assessment, and
accuracy firings.
2-10 COMPLETE DESIGN OF GUN,
ROUND. AND ANCILLARY EQUIP¬
MENT
As the design, development, and manufac¬
ture of a recoilless rifle weapon system are
beyond the expertise of any single organiza¬
tion, it is the responsibility of the developing
agency to coordinate separate developments
of the various components of the weapon
system. It is necessary to coordinate the
design and development of such gun compo¬
nents as the mount and such ancillary
equipment as the spotting rifle in order to
ensure that these parts have met their
respective requirements. Since these compo¬
nents are not integrated into the system until
prototype units are made, it is important that
the component requirements be compatible.
Part Three of this handbook deals with the
design of the components that make up a
recoilless rifle weapon system. Chapters 10
through 13 indicate both the considerations
to be nude in designing the rifle, ammunition,
mount, and fire control device, respectively,
and how the component design affects or is
affected by the design or performance of the
other components. Only in light of the design
and performance of the other components
will the principal components integrate to
produce the desired product.
SECTION IV
NUMERICAL EXAMPLE,
The numerical example that follows is
based on the procedures outlined in par. 5-20
of Chapter 5 for determining the peak
pressure to be used for obtaining the
minimum weight gun. The calculations are
performed using requirements for the 120
mm HAW Weapon System. Explanation of
the various parameters are contained in
Chapter 5.
1. Given Constants:
a. ?. pellant :
1/p » 17.09 in.’-lb: 1
k * 6.46 X 10’* sec" 1
F - 3.3 X 10* (fHbHb-*
b. Weapon System :
A * 17.393 in. 2
4. Solution:
4>' m * 2/ V m , (fpsf 1 for minimum weight
gun (Eq. 5-64)
- 1.104 x io- J (fpsr 1
/ (V m , X) * kf/(A/A t ), dimensionless
_ (1.104 X 1 Or 8 ) (6.46 X 10r>)(3.3X 10 s )
1.46
* 1.638
From Fig. 5-18 for/* 1.638,
X - 0.53
Estimate Value of C (
C.„* 9.601b
M 18.1 ,
m *— * —— * 0.562 slug
g 32.2
A, *11.911 in. 2
A/A» 1.437
M « 18.1 lb
m * 1.04 m +
.04 |«
(1 -
3g
O.i
- 1 04 0.562 +
Hfi] .slug
3) (9.60 )1
2 . 2 ) J
.(1-0.53)
3(32.
V m * 1810 fps
» * 0.3 lb/-in." 3
a - 160,000 psi
/?, - 2.35 in.
2. Chosen Constant: P p * 10,000 psi
3. Assumptions:
a- y b - y m
b. No ' o
* 0.63 slug
From Fig. 5-17 for P p * 10,000 psi
B * 6.40 X 1(T* in.^pa-sec)' 1
W 0 * m'BV m IA, in.
* (0.63) (6.40 X 1CT*) (1810)
- - Tr3§r —^
* 0.042 in.
C, * kA t W g l(kB), lb (from definition
of X)
2-17
AMCPMMSt
» (6.46 X IQ-*) (11.911) (0.042)
(0.53) (6.40 X 1<T)
* 9.53 lb
From Fig. 5*19 for ij/' * 1.104 X 10“*
(f^r 1
- l.o x icr 3 (fpsr*
For minimum weight gun
12 m'exp(\|>' V ) , .
*-• n ^
For minimum weight gun:
?m “ 4>' m pe 2
(since 4>m * 2)
w < 10,000X1 OX 10~* )*(1810)(2.718)
(1.104 X 10-*) (2.718)*
P m " 6031 pci
Using a safety factor of 1.15:
where
i-' * Cj IpA )and 4>' m V m a 2
x m 12 (0.63) (2.718)* _
" * (10,000)07.393)0.0 X 10'*)*(2.178)
(9,60)07.09)
+ 17.393
ot^ * 127.6 in.
For minimum weight gun:
a _ 12(0.63) _
* (10,000)(P.393)(!.OX icr')*
(9.6)07,09)
+ 17.393
- 52.9 in.
•v * ■ — - + —A
° P p AW p )'e pA
12(2.7 1 8)* (0.63) _
(10,000X17. - 0(2.718X1.0 X lO" 3 )*
(9.60) (17a >)
+ 17.393
» 2S.4 in.
-Obtained from tq. S4I bated on (be eiaimption that q
• 0 fat a minimum weight weapon.
p‘ m i i sp„
P P
P' p - 1.15 X 10.000* 11.500 psi
" ! l5/» w .psi
P' m * 0.15) (6031)* 6936 psi
Calculating the wall thicknesses by Eq.
5-109 corresponding (o P' p and/^
s ?p Hilo. in.
01. S00M 2.35)
160,000
• 0.169 in.
w ’m * P' m Pi ■ 0 • ' n -
* (6936) (2.35) _
160.00(5 -0.10. in.
The weight ot the gun then is approxi¬
mated by Eq. 5-112 and noting that it was
assumed \\ = I' . thus a* * a* . Also Eq.
5-112 shows that the tube weight and the gun
weight are estimated by adding the chamber
weight to Eq 5-112.
v ,u. -2w’K,r<»„
-*>]
+ »P'[ K p 1*0 + V
2-18
AMCP7M-238
+ (»l * w p w m * <>
x ( i T i )]-‘ b
- 2*(0.3)(2.35)[(25.4 + 52.9)
X (0.169) + 12iM±0d02)
2
X (127.6-52.9)] +*(0.3)
j (0.169) 1 (25.4 + 52.9)
+ [(0.169) 1 + (0.169) (0.102)
+ ( 0 . 102 ) 1 ]
» 104.81b
27,7(9.60)
17.393(25 4)
0.60 g-cnf*
Repealing these calculations for different
values of P . a curve of bare rifle weight
versus peak pressure for X « 0.53 is generated
as shown in Fig. 2-9. Examining the loading
density A 0 for the various peak pressures, we
find the following:
*V.pa
A,**"
8,000
0.518
9,000
0.562
10,000
0.600
11,000
0.636
12,000
0.670
14,000
0.731
From the discussion of par. 2-8, it is shown
that the appropriate choice of P p would be
10,000 psi in order to attain the desired
loading density ef 0.6 g-cnf 1 . The propellant
charge for X * 0.53 is 9.6 lb. In the final
design of the 120 mm Rifle. XM105. HAW.
the following parameters were obtained
experimentally for a muzzle velocity of 1800
fpsat 70°F:
P = 10,300 psi
C, = 9.5 lb
These values are extremely close to the
calculated values obtained for the minimum
weight gun. Given the optimum values of P p
and X for a minimum weight gun. it is possible
to determine all the rest of the gun and
propellant parameters as well as the pressure-
travel relation through the method of solving
the interior ballistic equations as described in
Section V of Chapter 5. An example of these
remaining calculations is given in par. 5-19.
M9
Pare Weapon Weight, lb
AMCP 708-238
Peak Pressure, KSI
Figure 2-9. Bare Weapon Weight vs Peak Pressure
2-20
AMCP 706-23B
REFERENCES
1. ST 9-152, Ordnance Technical Terminolo¬
gy, US Army Ordnance School, Aberdeen
Proving Ground, Maryland, June 1962.
2. AMCP 706-150, Engineering Design Hand¬
book, Interior Ballistics of Guns.
3. PDWL S-2, Notes on Development Type
Materiel, 120mm Rifle System XM105E1
Heavy Antit ink Weapon (HAW), Frank-
ford Arsenal, December 1962.
4. R.T. Fillman and D.E. Walters, Large
Caliber Recoilless Cannon (EIK), Report
R-1533, Frankford Arsenal, March I960.
5. Recoilless Rifle Systems, Ammunition and
Related Items, Status Report No. 1, Vol.
II, Report R-1237, Frankford Arsenal, 1
Or*ober through 31 March 1954.
6. Development of the 90mm Rifle T149,
Interim Technical Report, Contract No.
DA-19-020-ORD-40, Arthur D. Little,
Inc., June 1, 1955.
2-21
AMCP 706-238
PART TWO
THEORETICAL ANALYSIS
CHAPTER 3
TERMINAL BALLISTICS
3-0 LIST OF SYMBOLS
A - constant of Eq. 3-1, dimensionless
A = average presented area of fragment,
ft 2
B = constant of Eq. 3-5, g 1/2 -in. “ 7/6
C = explosive charge weight, g
C D = average drag coefficient, dimen¬
sionless
d t = projectile inside diameter, in.
d 0 = projectile outside diameter, in.
e = base of natural logarithm
2.7 , 8281828. . .
y/7F = Gurney constant, fps
G = number of grooves per ring
K = constant of Eq. 3-9, lb-ff 3
A/ = total weight of projectile, g
m = fragment weight, g or lb
N(m) = total number of fragments of weight
greater than m
N Q = total number of fragments
R - outside radius of case, in.
t = wall thickness, in.
V = velocity of fragment, fps
V a - initial fragment velocity, fps
W = mean width between grooves, in.
x = distance from point of burst, ft
M = quantity in Mott equation related
to average fragment mass, g
0 = angle measured from nose of pro¬
jectile, deg
p(fl) = fragment density, fragments per
solid angle (steradian)
p a = weight density of air, lb-ft” 3
p c = explosive charge weight density,
lb-ff 3
p m = metal case weight density, lb-ff 3
3-1
AMCP 706-238
SECTION I
INTRODUCTION
3-1 SCOPE
Warheads used in recoilless rifle weapon
systems are similar to the warheads of conven¬
tional artillery ammunition. Since the same or
similar types of explosive charge material,
fuzes, projectile material, etc., are the same
for recoilless and conventional artillery am¬
munition, this information is not presented
herein and the reader is directed to the
material on terminal ballistics contained in
Refs. 1 and 2. This chapter will direct its
discussion to the factors that affect the
terminal ballistic performance of recoilless
rifle ammunition.
3-2 BACKGROUND
Terminal ballistics is concerned with the
principles underlying the effects of weapons
on targets. The effects studied include pene¬
tration, fragmentation, detonation, sliaped
charge, blast, combustion, and incendiary
effects. Because these effects are dependent
upon the firing and flight characteristics of
the projectile, the terminal ballistic study
includes all actions of the warhead from
safing and arming to effect on the target.
In designing weapons and ammunition,
maximum desired terminal effect is a primary
objective. In order to achieve this objective, a
proper balance of many factors is essential.
The most important of these factors are
shape, weight, and material used in the
projectile; type and weight of explosive
charge; fuzing system; and terminal velocity.
In order to evaluate or determine the scaling
of these parameters, various experiments are
conducted to detennine the principles govern¬
ing the number, size, velocity, and spatial
distribution of fragments resulting from deto¬
nations of cased high explosive charges. These
experiments also include the study of penetra¬
tion, impact, blast, and shaped charge ef-
fects-depending upon the type of warheads
under study. The basic information provided
during these terminal ballistic studies and
experiments permits the optimization of the
parameters and effects on particular types of
targets. These studies also provide the infor¬
mation required to evaluate the overall system
effectiveness. For example, terminal ballistic
studies, i.e., distribution of fragments by size
and velocity, provide the data needed to
determine kill probability.
3-3 TYPICAL RECOILLESS WARHEADS
Recoilless rifles a*e large caliber weapons of
light weight, great striking power and accu¬
racy, delivering no recoil to its mount or, if
shoulder-fired, to the body of the individual
tiring the weapon. These characteristics make
the recoilless rifle ideal for infantry attack
against heavily armored vehicles such as tanks.
Most rtsoilless rifle weapon systems, be¬
cause of their light weight and relatively low
muzzle velocity, cannot function effectively
when employing AP (armor-piercing) type
warheads that rely totally on the kinetic
energy of the projectile to enable it to
penetrate the target armor plate. In normal
combat conditions, complete incapacitation
of armored veiiicles is not necessary to put
the vehicle out of action. Various components
and moving parts such as controls, engine, gun
and running gear can become inoperable by
being wedged, burred, deformed, or cut off to
cause immobilization or uselessness of the
vehicle. This can result projectile fragments
and blast on the outside of the vehicle or by
spai) particle** on the inside. Furthermore, a
3-3
Preceding page blank
AMCP 706-296
hit on vital components such as ammunition,
and sometimes fuel, can cause vehicle destruc¬
tion b> fire.
Because HE (High Explosive), HEAT (High
Explosive Antitank), and HE? (High Explo¬
sive Plastic) warheads do not rely exclusively
on total armor penetration to defeat the
target-i.e., they do not require a high muzzle
velocity, but rely on fragmentation, blast, or
spalling characteristics to defeat the target-
they are used effectively in recoilless rifle
weapon systems against armored targets. Figs.
3-1 and 3-2 show the typical configuration of
HEAT and HE recoilless warheads, respec¬
tively.
High Explosive
Charge
/
Figure 3-1. Typical HE A T Recoilless Warhead Cross Section
3-4
AMO* 706-238
SECTION II
HEAT WARHEAD
34 QUALITATIVE. DESCRIPTION
The HEAT warhead is a special type of
high explosive warhead that incorporates a
shaped charge. The taic components of the
shaped charge warhead are container, hollow
liner of inert material, fuze, explosive charge,
and detonating device. As shown in Fig. 3-1,
the shaped charge warhead has an axially
symmetric high explosive charge positioned
behind an inert liner in the form of a cone
with its apex toward the detonator. In opera¬
tion, the high explosive charge illustrated is
initiated by the impact of the warhead which
generates a current in the piezoelectric crystal
that, in turn, functions the fuze and detona¬
tor. The generated shock wave in the explo¬
sive collapses some of the liner material into a
high velocity stream of metal called a jet. The
forward end of the jet attains a velocity
approaching the detonation velocity of the
explosive (25,000 fps) while the aft end of
the jet and the remaining liner material (called
the “slug”) have a forward velocity of about
1500 fps. If the liner material is sufficiently
ductile and there is sufficient space, the liner
will be drawn out into a v;ry long thin jet of
extraordinary penetrating ability. The dis¬
tance between the base of the liner and the
surface to be attacked is called the “standoff'
and depending upon the type of charge, liner
material, and other parameters, there will be
an optimum standoff for which the greatest
penetration is achieved against the specific
type of target.
Against armored vehicles, the damage in¬
flicted by a HEAT warhead stems from the
ability of the jet to penetrate the armor
thickness and from the production of spalls
on the opposite side of the armor surface
under attack. Property designed shaped
charged warheads detonated at the optimum
standoff can penetrate thicknesses of steel
armor equal to three or four time? their
conical diameter.
For detailed quantitative information on
the resistance of armor against HEAT rounds,
see Ref. 3.
35 FACTORS AFFECTING PERFOR¬
MANCE
3-5.1 INTRODUCTION
The performance of shaped charge war¬
heads can be evaluated by several methods
that determine the an ount or type of pene¬
tration in a homogeneous reproducible type
of material. In most cases and in the material
contained within this chapter, the measure of
performance is taken as the total depth of
penetration into mild steel, except where
otherwise stated. The equivalent penetration
in homogeneous armor is obtained by multi¬
plying by a conversion factor. Homogeneous
armor generally is not used as a target
material because of its much greater cost than
mild steel. Fortunately, different grades or
types of mild steel all give about the same
average penetration for a given shaped charge
design. In measuring the depth of penetration,
targets often are made up of stacks of mild
steel plates 0.5 to 3.0 in. thick.
In various tests as reported in Ref. 1, it is
found that the penetration of a given jet into
steel at a fixed standoff varies essentially
linearly with the Brinell hardness of the steel
and that the penetration also is affected by
the standoff. Tests also indicate that in
comparing the relative performance of the
ability of a given shaped charge to penetrate
mild steel and homogeneous armor, the armor
is more effective than the mild steel in
3-7
AMCP 70C-Z3S
resisting penetration of a given jet at longer
standoffs.
For some purposes, a better measure of
performance would be the volume cf the hole
or its smallest diameter. The best measure of
performance, especially when considering the
lethality of the warhead, would be the
measurement of some factor that indicates
the amount of damage done behind a given
target plate by the residual jet and spalled
material from the back face of the plate.
3-5.2 PROJECTILE SPIN
One of the problems or disadvantages
encountered in the use of shaped charges is
that rotation of the warhead in spin-stabilized
projectiles reduces the penetration capability
of the jet. Increasing standoff increases this
effect. Fig. 3-3 shows the relation of penetra¬
tion to rotational speed, and indicates the
undesirable effect of rotation on penetration.
The reduction in penetration caused by rota¬
tion is attributed to the lateral dispersion of
the jet which results in decreasing the effec¬
tive mean density of the jet.
Attempts have been made to improve the
performance of spin-stabilized HEAT prqjeo-
tiles by using noncomcal, axially symmetric
liners. However, at high spin rates, the results
Figure 33. Penetration as a Function of Projectile Spin Rate (Ref. 1)
3-8
AMCP 7 Ob-238
were not promising and the major emphasis
shifted to the design of fluted liners not
having axial symmetry. The principle under¬
lying the use of fluted liners is that of “spin
compensation”, i.e., of conserving the angular
momentum of the liner so as to inhibit the
dispersion of the jet. Generally, spin compen¬
sation is achieved by using flutes that arc in
the plane of the charge axis and that have an
increasing thickness as one proceeds from the
apex to the base of the cone. However, the
mechanism of spin compensation is not yet
fully undeutood, since even the direction of
spin compensation can be reversed in some
cases by just changing the number of flutings.
For the present, it is necessary to rely on
available empirical data when considering the
use of spin compensation.
3*5.3 PHYSICAL PROPERTIES OF LINER
The formation of a shaped charge jet from
the collapsing cone is a critical process and
can be affected adversely by deviations in the
required geometrical accuracies and metallur¬
gical properties of the liner. In order for the
walls of the cone to collapse and meet exactly
on the axis of the cone, several geometrical
requirements must be satisfied or the sides of
the cone will not collapse uniformly on the
cone axis and will produce a crooked jet that
wanders at the point of contact on the target
surface, resulting in poor penetration. Thus, it
is important that sections of the cone perpen¬
dicular to the cone axis be true circles with
centers on the axis and that the walls be of
uniform thickness around the section circum¬
ference. Uniform density of the metal also is
required for even collapse of the walls. The
last undesirable characteristic is the existence
of waviness along the slant height of the cone.
Deviations in the metallurgical properties
of the liner can result in the same reductions
in the effectiveness of the warhead as caused
by the geometrical inaccuracies of the liner
previously discussed. Metallurgical properties
of the liner depend strongly on the manufac¬
turing method and type of heat treatment.
Because of the extremely high pressures, high
strain rates, and excessive amount of plastic
strain involved in the collapse of the liner, it is
difficult to analyze the metallurgical state of
the liner in all states of jet formation. Also it
must be remembered that the properties of
the jet and the cone are not the same, and
that the most important properties are those
considered under the high pressures and rales
of strain previously mentioned. These proper¬
ties may be very different from those under
ordinary conditions, as emphasized by the
fact that glass cones give penetrations in
concrete targets greater than might be ex¬
pected from the metallurgical properties of
glass. Some very interesting and important
correlations exist among properties of the
liner, principally crystal structure and melting
point, and behavior of the jet. One of the
most interesting features is a built-in spin
compensation factor in certain cases, appar¬
ently resulting from an unusual crystal struc¬
ture as a consequence of a particular forming
process that would give less penetration in
static firings.
Theory indicates that the penetration of
the jet is proportional to the length of the jet
and the square root of the jet density. For a
continuous jet the assumption is made that
the jet density is the same as that of the cone.
As a result of the velocity gradient in the jet.
the jet lengthens as it travels and, because of
this stretching, eventually breaks up into a
series of particles. If the jet did not break up,
its length and penetration would increase
linearly with time and, consequently, with
standoff.
Actual data indicate that penetration in¬
creases with standoff up to a maximum value
of penetration. The standoff corresponding to
this maximum penetration is called the “opti¬
mum” standoff. Beyond the optimum stand¬
off, the average penetration decreases with
increasing standoff, while the best values of
penetration approach an asymptotic value.
3-9
AMCT 7tt-2M
The decrease in penetration from the idea)
liner value to the asymptotic value is due to
the breakup of the jet, whereas the decrease
in penetration from the asymptotic value to
the average value is due to increasing spread
of the jet. Thus, for good penetration, the jet
should be capable of attaining a great length
before breaking up. The ability of the jet to
attain the desired lengths depends upon its
metallurgical properties, homogeneity of the
explosive filler, and the accuracy of compo¬
nent manufacture and assembly. In tests
comparing the penetration capabilities of cop¬
per, aluminum, steel, zinc, lead, and glass
liners, it is found that copper and aluminum
have the best metallurgical properties for
shaped charge cones while lead and glass have
inferior properties. A desirable liner material
would have properties similar to copper and
aluminum, and have a high density (Ref. 1).
3-5.4 STANDOFF
Fig. 3-4 shows the relationship between
penetration and standoff for copper cones
into mild steel targets under test conditions.
As indicated in Fig. 3-4. the maximum pene¬
tration would occur at a standoff of about 6
cone diameters. In reality, the actual standoff
for a well-made conical liner would be limited
to one to three cone diameteis by such
aerodynamic considerations as ogive shape
and size, and projectile velocity. However,
these shorter standoffs may be sufficient to
attain 80 to 90 percent of the penetration
expected at optimum standoff indicated in
Fig. 3-4.
A properly designed cone will achieve the
required level of penetration while exhibiting
a fairly fljt penetration-standoff curve, i.e.. an
w
t-
0 )
o
6
S
T3
0)
C
o
o
•k
c
o
2
<5
c
0 )
a,
rigure 3-4. Penetration for 3C-deg Eiectroformed Copper Cones into Mild Steel Targets
(Ref 1)
3-10
amct 7»a
increase or reduction in standoff will not
change the penetration greatly. Both alumi¬
num and copper liners exhibit fairly flat
penetration-standoff curves, as compared to
steel liners which exhibit a sharply decreasing
penetration after peaking at a small optimum
standoff.
3-5.5 CONE ANGLE
The choice of the conr apex angle is
important, both from a performance and a
manufacturing standpoint. Data are available
that indicate the optimum standoff increases
with increased apex angle up to about 65 deg:
optimum standoff or maximum penetration
then decreases as the apex angle is increased.
However, the optimum standoff is also depen-
den 1 upon the cone material, wail thickness,
and charge length.
With modem, precision-manufacturing
methods, the optimum cone angle for projec¬
tiles with copper cones is close to 40 to 45
deg. However, certain cases, as indicated in
Fig. 3-5, have shown best penetration perfor¬
mance with 20-deg cones, and in others.
60-deg cones. As a first choice, a cone angle
•o
CO
u
o
4 -»
0)
E
«
a>
c
o
0
c
o
4)
c
0)
CL
Figure J-5. Maximum Penetration into Mild Steel Targets a: Optimum Standoff vs Cone
Ar?.fe i‘o, Electro formed Copper Cones (Pef. 1)
3-11
AMCP 70*231
of either 40 or 45 deg may be selected and
will give good performance in projectiles with
an ogive length of 2 calibers (Ref. 1).
As with most other cone parameters, the
effect of the cone apex angle decreases with
increasing projectile spin rate. For example, at
0 rps a 45-deg, 3.4-in. copper cone penetrates
3 in. deeper than a 69-deg cone of the same
wall thickness; but at 45 rps, the difference is
less than 1 in.
3-5.6 LINER WALL THICKNESS
For each type of cone material, standoff,
projectile wall confinement, explosive type,
charge shape, and cone apex angle, there is an
optimum wall thickness. From a practical
consideration of projectile design, the projec¬
tile confinement and cone apex angle are the
most determining factors.
As a guide for liners of different apex
angles, or for shapes other than conical, an
approximately correct wall thickness may be
obtained by maintaining the thickness con¬
stant in the axial direction. As shown in Fig.
3-6, curves of penetration versus wall thick¬
ness are frequently unsymmetrical. A thicker
wall generally is preferred over a thinner wall
since thin-wall performance is typified by
excessive variability from charge to charge,
whereas the thicker-wall performance is char¬
acterized by good reproducibility with only a
tolerable decrease in penetration. In practice,
a wall thickness about 5 percent greater than
the optimum is selected in order to insure
that the production wall thickness will not be
less than optimum (Ref. I).
Various studies have indicated that the
liner thickness should scale as the diameter,
i.e., a cone would logically be thicker at the
base than at the apex. The investigations of
tapered walls, however, have shown that the
real improvement in the penetration perfor¬
mance is slight, if any at all. These studies
have indicated, however, that rather wide
tolerances may be placed on the variation in
wall thicknesses between apex and base with¬
out reducing penetration, provided the wall
thickness is held constant at each transverse
section of the cone.
36.7 LINER SHAPE
Nonconical shapes that have been tried as
liners-in addition to the simple cone already
described-are hemispheres and spherical caps,
trumpets, and combinations of these. The
general results are that the penetrations
achieved from these configurations are infe¬
rior to those obtained with simple cones.
Radiographs show that hemispheres do not
collapse with the formation of a jet, as do
cones; they tum inside out before collapsing,
with the whole liner being projected as a
stream of particles. Spherical caps (segments)
are fragmented and projected as a cluster of
particles that may be more or less focused
depending upon the curvature. Results ob¬
tained from the use of spherical segments
show poorer results than those obtained with
hemispheres.
Double cone angles in which there is a
change from one angle to another have shown
.jood performance in certain cases. When the
change in angles is made abruptly, there has
been no evidence of any increase in penetra¬
tion. However, when the change in angles is
made smoothly and the liner wall tapered,
rounds have given peak performance at nor¬
mally available standoffs.
35.8 ALIGNMENT OF CONE AND
CHARGE
For the best and most reproducible perfor¬
mance, the axes of the charge and cone
should coincide. In actual practice, however,
the axes may not be parallel (tilted), or they
may be parallel but displaced (offset). Tilting
of the liner results in a reduced average
penetration. Although it is possible to obtain
good shots with liners tilted as high as 2 deg.
3-12
AMCP 706-238
Cone Thickness, in cone diameter (c.d.)
Figure 3-6. Cone Thickness vs Penetration for45-deg Copper Cones (Ref. 1)
the general findings are that a 1-deg tilt of the
cone reduces the average penetration by 50
percent, a 0.5-deg tilt by 20 percent, and a
0.3-deg tilt by 10 percent.
The second type of misalignment results
when the cone and charge axes are parallel
but slightly offset. Actual tests have shown
that an offset of only 0.015 in. (1 percent of
the base diameter) reduced the penetration by
approximately 20 percent.
3-5.9 CONFINEMENT
Increasing the confinement of the jet either
3-13
AMCP 706-238
by providing an increased wall thickness or a
“belt” of explosive greatly increases the hole
volume of the penetration. The presence of
explosion products at high pressure within the
explosive belt retards the expansion of the
products in much the same manner as does a
steel casing. In the study of the effects of
confinement on the performance of flanged
and unflanged cones (Ref. 1, par. 2-93) the
following conclusions have been drawn:
1. The addition of a small explosive belt
obtained by increasing the charge diameter
from 1.73 to 2.00 in. produces the same
effect on penetration and hole volume for a
1.63 base diameter cone as the addition of
0.25 in. of steel confinement.
2. When heavy base confinement is added
to the 2-in. charge, the penetration is de¬
creased about 20 percent.
3. The addition of both lateral and heavy
base confinement to the 2-in. charge causes a
drastic reduction of about 45 percent in
penetration performance.
4. When the larger charge is confinH
faterally, the presence of a flange causes a
relatively small, but significant decrease in
penetration, as compared with a similarly
confined charge lined with a deflanged cone.
5. The hole volume produced by the 2-in.
charge is increased by about 50 percent when
lateral confinement of 0.25-in. steel is used
(compared with the 100 percent increase
which occurs with the 1.63-in. charge);
boundary conditions at the base of the charge
have little or no effect on hole volume in spite
of the large changes in depth of penetration.
3-14
AM CP 706-238
SECTION lil
HE WARHEAD
3-6 QUALITATIVE DESCRIPTION
The HE warhead consists of a high explo¬
sive charge and fuze surrounded by either a
wall of preformed metal fragments or a
prescored or solid metal casing. Upon detona¬
tion of the high explosive, the metal case
expands with the fragments being propelled
outward at velocities of 6,000 to 10,000 fps.
In effect, the. fragments are projectiles with
the capacity to inflict considerable damage to
adjacent objects. Capacity for damage de¬
pends upon fragment size, shape, velocity,
and distribution.
Fragmentation is not the only result of
detonation of the HE warhead. Approxi¬
mately forty percent of the gas energy nor¬
mally is expended in the fragmentation
process with the balance of the available
energy being consumed in the formation of a
compressive wave in the air surrounding the
projectile.
3-7 DETERMINATION OF FRAGMENTA¬
TION CHARACTERISTICS
3-7.1 FRAGMENT SIZE DISTRIBUTION
Mott and Linfoot (see Ref. 1) proposed
that the fragmentation of thin-walled projec¬
tiles is the result of two-dimensional, rather
than three-dimensional, breakup. Based on this
assumption, the mass distribution of frag¬
ments may be described by the equation
N(m) = A exp ( — m/V) 1/2 (3-1)
where
N(m) = total nuinuer of fragments of
weight greater than m
m = fr; gmen t weight, g
H = function of average fragment
weight m, g
H = m/2
A = constant
If it is assumed that the two-dimensional
breakup holds down to the smallest fragment,
then
N(m) = lM/(2*i)] exp ( - m/p) i/2 (3-2)
where
M - total weight of "rojectile, g
2n = arithmetic average fragment weight,
g
Noting that A//(2p) represents the total num¬
ber of fragments N 0 , Eq. 3-2 also may be
written
N(m) = N 0 exp ( — m/h) 1/2 (3-3)
For extremely thick-walled projectiles, the
wall thickness will have less effect on the size
of the fragment. Also, three-dimensional
breakup rather than two-dimensional breakup
will be the rule. The weight distribution of
fragments for this case is described by
N(m) = A exp ( - m/n) i/3 (3-4)
For fragmentation projectiles, Eq. 3-3 is more
representative than Eq. 3-4 of the conditions
found.
If one assumes the validity of the Mott
equation, the quantity n is a measure of the
fragmentation efficiency of the projectile and
is dependent both upon the characteristics of
the explosive and of the metal case. The
AMCP 706-238
significance of the quantity is made clearer by
stating that the number of fragments greater
than tig is equal to the number of fragments
having masses between til 11 and /i g. Thus, if
ti = S.S g, the number of fragments lying
between 0.S and S.S g would be equal to the
number of fragments with weight greater than
S.S g (assuming that the Mott equation is
valid down to fragments as small as 0.S g).
Furthermore, if the Mott equation was valid
for all fragments, then the number of frag¬
ments greater than p would comprise 37 per¬
cent of the total.
following formula, relating the value of ti to
the projectile inside diameter d t and the wall
thickness t has been proposed by Mott:
M 1/2 = Bf 5/6 di /8 (l + t/d { ) (3-6)
where
B = constant depending upon the explosive
and the physical characteristic of the
metal of the casing
dt = projectile inside diameter, in.
As stated in Ref. 1, there has been diffi¬
culty in analyzing existing test data because
of the nonuniform behavior of the projectiles.
Even within a single lot of projectiles, there is
considerable variation in the number of frag¬
ments produced by the individual warheads.
Thus ; a. rough agreement between exist¬
ing experimental data and the semitheoretical
formula developed by Mott exists. A series of
experimental firings with steel projectiles
filled with explosives of different character¬
istics was performed at the US Naval Ord¬
nance Laboratory in order to obtain values of
the parameter m as well as other parameters
and characteristics of the fragmentation
process. Plots on semilogarithmic paper of the
cumulative number of fragments versus the
square root of the fragment mass were ob¬
tained with several representative plots shown
in Fig. 3-7. The Mott equation predicts a
straight line of these plots. However, as shown
in Fig. 3-7, it ct.n be seen that the experi¬
mental points in every case form a curve of
increasing negative slope rather than a straight
line. Assuming that this experiment was accu¬
rate, it would seem to indicate a fundamental
defect in the Mott relationship.
The value of in addition to being
dependent upon the characteristics of the
explosive and projectile material, also is de¬
pendent upon the physical dimensions of the
projectile. To account for this variability,
scaling formulas have been proposed. The
t = wall thickness, in.
2-7.2 INITIAL FRAGMENT SPEED
The initial speed V Q of fragments is pre¬
dicted quite accurately by the following
formulas developed by Gumey:
1. For cylinders:
• ** (3 - 6)
2. For spheres:
Vo
c/m y /2
+ 0.6 (C/M)J '
fps (3-7;
\fZE~ = Gumey constant for each type of
explosive, fps
C = weight of explosive charge, g
M = weight of fragmenting metal, g
Table 3-1 gives the value of \/ lE t Gumey
constant for most of the commonly used high
explosives.
The graphs in Fig. 3-8 simplify the calcula¬
tions for V Q in terms of the outside diameter
d Q and thickness t of the projectile, the ratio
3-16
Cumulative Number of Fragments Cumulative Number of Fragments
3-1
AMCP 706-238
TABLE 3-1
GURNEY CONSTANT FOR VARIOUS
EXPLOSIVES (Rtf. 1)
Exploits
Gurnay Constant
V2f. <P*
Composition C*3
8,800
Composition B
8,800
Torpex 2
8,800
Composition H-6
8,400
Pentolite
8,400
Minol 2
8,300
HBX
8,100
TNT
7,600
Tritonal
7,600
Picratol
7,600
Baratoi
6,800
p c lp m of the density of the explosive to that
of the metal case, and the Gurney constant of
the explosive. Knowing the value of tld 0 , one
uses Fig. 3-8(A) to solve for (C/M)/(fi c /p m ).
The value of CIM may then be found by
multiplying by p c lp m . This value of CjM then
may be used to find V 0 f^/2E from Fig.
3-8(B). Multiplying this expression by the
appropriate value of \/TE from Table 3-1
gives the value of the initial fragment speed
Vo-
3-7.3 FRAGMENT SLOW DOWN
The equation for the velocity V of a
fragment at a distance x from the point of
burst is given by the following relationship:
V = V 0 exp , fpc (3-8)
where
V - speed of fragment at x feet from
point of burst, fps
V 0 = initial speed of fragment, fps
C D = average drag coefficient, dimension¬
less
A = average presented area of fragment,
ft 2
p a = density of air, lb-ff 3
x = distance from point of burst, ft
m = weight of fragment, !b
For any homologous class of regularly
shaped fragments, the weight m and average
presented area A are related by the following
equation:
m=K(A)* n (3-9)
where
K = constant for the particular class of
projectile
The value of K has been shown by experi¬
mental results to be roughly constant for the
fragments projected from a particular projec¬
tile. Values of K are given in BRL Reports
501, 536, and M915 for a variety of projectile
an.’ bomb fragments.
The method used to determine the pre¬
sented area of the fragment involves measur¬
ing the presented area of the fragment for
each of 16 positions corresponding to the
orientation of 10 of the 20 faces of an
icosahedron plus 6 orientations corresponding
to the 12 vertices of the icosahedron. The
arithmetical average of these values is then
used for .4. The instrument used to obtain the
presented area is known as an icosahedron
gage.
3-/A FRAGMENTATION PATTERNS
When a projectile or warhead bursts, frag¬
ments are projected in different directions
depending upon the configuration of the
projectile. If the projectile were spherical and
stationary when detonated, the density of
fragments would be substantially constant,
3-18
AMGP 706-239
regardless of the direction. If the projectile
were entirely cylindrical, the greatest density
of fragments would be close to the equatorial
plane, with practially all fragments contained
in a narrow side-spray of the order of 20-deg
width. For an ordinary artillery projectile, the
curve of distribution with angle is peaked, and
resembles the “normal error curve’*. An ex¬
ample is shown in Fig. 3-9.
Projectiles and warheads almost always
have circular symmetry about their longi¬
tudinal axis. Hence, the distribution of frag¬
ment mass and velocity may be described as
functions of the angle 0 measured from the
nose of the projectile (see Fig. 3-9). Letting
p(0) be the fragment density in fragments per
unit solid angle, the total number of frag¬
ments N 0 of the given projectile is given by
N 0 = 2* j* p(0) BinOdO (3-10)
3-7.6 CONTROLLED FRAGMENTATION
In uncontrolled fragmentation, the range of
masses and speeds is very great. In order to
secure more effective fragments, it is desired
to solve for the optimum mass (depending on
the lethality requirements) and design a
projectile that would limit all fragments with
this mass. Thus, the probability of damage
would be greatly increased and the results
estimated more correctly.
The methods for controlling fragmentation
are described in the paragraphs that follow.
3-7.5.1 Preformed Fragment
The best method of controlling fragment
size is to form or precut the fragments to the
desired size before incorporation into the
projectile wall. The projectile structure usual¬
ly is formed by a thin metal liner or cover, or
4)
c
<
o
CO
c
p
L
&
a
5
2
£
cm
Figure 3-f Typical Angular Fragmentation Distribution (Ref. 1)
AMCP 706-23*
\
i
both, to which the fragments are fastened
with adnesive. Another method of installing
the preformed fragments, especially spherical
or fln-»iabilized fragments, is to place the
fragments i. layers between the inner liner
and the cjl and then fill the crevices between
them with a plastic matrix.
£-7.5.2 Notched or Grooved Rings
In this method of controlling fragmenta¬
tion, a series of notched rings are fitted over a
plastic or thin metal liner, each ring forming a
section of the warhead perpendicular to the
axis of symmetry. The forces from detonation
operate mostly in the direction of stressing
each ring circumferentially and only secon¬
darily to separate adjacent rings. Essentially,
the thickness and width of the rings provides
control of two dimensions of the fragments
while notches along the circumference of the
ring provide places of weakness where break¬
age in the third direction is desired.
The factors that are considered in this type
of projectile are:
1. Quality of steel in rings
2. Spacing of the grooves
3. Groove depth
4. Width of rings
5. Timer
6. Length-to-diameter ratio
7. Ring finish.
The material selected for the rings is of
relatively minor importance except that it
must be homogeneous. Test results to date
indicate that mild steel might be preferable to
high-carbon steel and that the steel should be
sufficiently hot-worked to break up segre¬
gated inclusions, and assure their uniform
distribution. The grooving spacing can be
determined from the following formula:
G = ir(2 R - t)/W (3-11)
where
G - number of grooves per ring
R = outside radius of case, in.
t = thickness of case, in.
W = mean width between grooves, in.
The depth of the groove should be from 5
to 10 percent of the ring thickness because
excessive groove depth causes the fragments
to break up. The grooves should have sharp
bottoms and the rings provided with a ground
or smooth lathe finish. In general, the width
of the ring should be made eqvai to the
thickness, the length to diameter ratio of the
case not less than 1.2S to 1, and the length
between 2.5 and 5 calibers.
The liner should be made of a material that
will produce no important fragments and
should be as thin as is consistent with
manufacturing and strength considerations. A
thickness of 5 percent of its radius has been
found satisfactory for laminated phenolic
plastic tubing.
3-7.5 3 Notched or Grooved Wire
in general, the notched wire method of
controlling fragmentation is similrr to the
notched rings. This method incorporates a
notched wire wound in a helix or spiral into
the shape of the warhead casing. The wire
must be supported by a liner or fastened
together by some means (such as welding) in
order to preserve the warhead shape. Notched
wires usually are used when a notch ring
would be too thin for economical manufac¬
ture.
3-21
AMQP 706-238
3-7.6.4 Notched Casings
Instead of notching in one direction and
having actual discontinuities in the metal in
the other direction (such as in the notched
ring or wire method), it is possible to cut,
punch, or cast a two-dimensional network on
a solid casing. Four types of notched casings
have been tested for their controlled fragmen¬
tation, namely (Ref. 1):
1. Cylinder, 4 in. O.D., 0.25-in. wall with
0.12S-in. holes in diamond pattern, punched
and plugged; holes, 0.5 in. apart in row, rows
0.5 in. apart.
2. Cylinder, 3.5 in. I.D., with linearly
tapered steps cut on outside, steps 0.5 in.
long.
■*. Cylinder 4 in. O.D., 0.25-in. wail, with
left-hand and right-hand helical grooves cut at
45 deg to axis and spaced 0.5 in. apart.
Groove profile V-shaped, with included angle
of 60 deg.
4. Cylinder, 4 in. O.D., 0.25-in. wall with
hexagonal pattern impressed by shearing.
Bach cylinder was *pprcximateiy 12 in.
long and eacn provided with brass endplates
*'j increase the confinement of the explosion.
In tests with these cylinders, only the hexa¬
gonal sheared pattern provided an excellent
degree of fragmentation control.
3-7.5.5 Multiple Walls
The multiple-wall projectile is made by
using close-fitted cylinders, each with thick¬
ness tjn where t is the thickness of a one-wall
projectile and n is the number of walls.
Multiple-wall projectiles do not give complete
control since only the thickness of the frag¬
ments is uniform. The number of fragments is
approximately n times the number of frag¬
ments of a single-wall projectile. The partial
control achieved, however, is an improve¬
ment, because the average fragment mass is
reduced and the number of fragments emitted
is increased. However, the increase in lethality
is much less than expected.
3-7.5.S Metallurgically Modified Material
Another method of fragmentation control
is to employ a type of iron, or a»> alloy, which
will fragment in a desirable fashion. For
example, peaiiite malleable cast iron provides
excellent lethality but its use is limited to low
setback items. Extrapolating from the cast
iron properties, a number of forgeable steel
alloys have been developed which yield in¬
creased Lihality solely as a result of their
composition.
3-22
AMC? 70*231
SECTION IV
OTHER TYPES OF WARHEADS
34 HEP WARHEAD
34.1 INTRODUCTION
The HEP warhead is used to defeat armor-
protected vehicles. The operational mode of
the HEP projectile is based on the fact that,
when a “sufficient” quantity of explosive, of
sufficient height for a given shape of explo¬
sive, is placed in intimate contact with armor
plate and detonated, a rupture of a portion of
the opposite face of the plate will occur. The
ruptured portion of the armor plate is known
as a spall and is generally in the form of a
rough disk. Dependent upon the quantity of
explosive above that needed to cause the
rupture, the spall may attain velocities be¬
tween 100 and 1000 fps. The mass and
velocity of the spall depends upon the quality
and thickness of the armor, and the mass type
and shape of the explosive filler.
In general, HEP warheads are designed to
defeat standard tank armor 1.2 calibers in
thickness at angles of obliquity of 0 to 60
deg. When considering weights alone, the HEP
projectile far surpasses the armor-piercing
projectiles in destructive power. Besides de¬
pending upon the armor thickness, the effec¬
tiveness of solid armor-piercing type projec¬
tiles is also higMy dependent upon the angle
of obliquity (the angle at which it impacts the
armor) and therefore must be designed to
penetrate a thickness greater than the actual
thickness. Since the HEP projectile shock
wave is transmitted normal to the armor
surface, the spall effect can be accomplished
on thicker plates than with a comparable
caliber, solid armor-piercing type projectile.
3-8.2 ADVANTAGES AND DISADVAN¬
TAGES
While not all the properties and character¬
istics of HEP type warheads are known, the
following characteristics and trends have been
observed:
1. Advantages are:
(a) HEP warheads make low-velocity
weapons, such as recoilless rifles, effective
antitank destroyers.
(b) While the effectiveness of other anti¬
tank projectiles decreases as the angle of
target obliquity increases, the effectiveness of
HEP projectiles decreases at a lower rate.
(c) HEP warheads are cheaper to manu¬
facture than other types of projectiles.
(d) The accuracy of HEP warheads is
comparable to or better than HE projectiles
fired from the same weapon.
(e) The blast and fragmentation from
HEP projectiles provide very desirable second¬
ary effects against primary targets (armored
vehicles).
(f) HEP warheads are cffc’ive in
neutralizing secondary' targets (iV-.vti ' Aic^s,
weapons, emplacements, personae! , .v' nor.-
armored vehicles).
2. Disadvantages are:
(a) HEP projectiles easily are defeated
by means of spaced or spiked armor.
(b) HEP projectiles have a low ballistic
coefficient because of their light weight and
blunt head shape.
(c) The plastic explosive filler of HEP
warheads must be press-loaded rather than
cast, taxing limited press-loading facilities.
3-23
AMCP
34L3 THEORY OF PERFORMANCE
When a charge of explosive is detonated in
contact with a flat steel plate, the explosive
energy is transmitted into the plate, normal to
the surface. The shock wave produced in the
steel is reflected from the rear surface of the
plate as another shock wave. The shock waves
meet at some line within the steel, and
reinforce each other, though not simply
additively, as with pure elastic stress waves. If
die charge is sufficiently great (the height and
shape of the explosive in contact with the
plate being important parameters), the steel
ruptures and a spall is driven off the rear side
of the plate. This action is a result of a
complex interaction of the reinforcement of
shock waves and the elastic stress waves.
The squashed charge of the HEP warhead is
most effective when it is in the form of a flat
cone. Since the explosive charge must adhere
closely to the surface and not break up, it
cannot be crumbly, but must have soft plastic
properties like putty which eliminates the use
of cast explosives. The spalling effect is best
produced with explosives that have a high
detonation velocity.
One of the more serious limitations of the
HEP warhead is its inability to function
satisfactorily outside a range of striking
velocities from approximately 1000 to 2000
fps. The maximum velocity limit exists be¬
cause deflagration of the explosive filler oc¬
curs when HEP warheads are fired at veloci¬
ties much above 2000 fps against armor plate
with 0-deg obliquity. The minimum velocity
limit exists because the functioning time of
existing fuzes and projectile crush-up on the
target are not properly coordinated at low
velocities. This problem is compounded at
low angles of obliquity because the projectile
tends to skid-off the target before function¬
ing. The minimum velocity limitation is a
serious handicap in the development of HEP
projectiles for some recoilless weapon sys¬
tems.
The fuzing requirments for HEP warheads
are that they should have sufficient delay
time in fuze functioning to allow for proper
projectile deformation. At high angles of
obliquity, the delay is shorter than for low
angles.
In the design of HEP warheads, •* has been
found that variations in nose material, nose
shape, nose length, nose hardness, and nose
thickness can have a marked effect on HEP
projectiles performance. Because the explo¬
sive shape at time of detonation is very
important in cai sing a spall, it was thought
that a softer nost like annealed copper would
be more suitable; but in actual tests, an
annealed steel nose gave better results. Also,
tests have shown that the blunt ogival nose, in
addition to giving better explosive effects,
also has better ballistic characteristics. In
addition to having an ogival shape, it is
preferable to have a long nose that will
provide a greater contact area upon impact.
Existing test data also have indicated that a
thin nose gives better results than a thick
nose.
3-8.4 GENERAL CONCLUSIONS
The following general conclusions have
been drawn from testing HEP warheads:
1. If the charge weight is held constant,
the weight of the spall displaced by cylindri¬
cal charges will increase as the charge
diameter is increased, up to the point where
the charge will have less than the minimum
thickness required to displace spalls.
2. The area of a displaced spall is usually
slightly greater than the area of the charge in
contact with the plate.
3. Explosive charges in the shape of a
conic frustum are more effective than an
equal weight of explosive in cylindrical shape.
4. The most effective shape of a charge is a
3-24
AllOP 709-23S
frustum of a right circular cone. An oblique
circular cone is not as effective.
5. Tough, ductile armor is spalled less
readily than higher strength, more brittle
armor. As the ductility of armor decreases,
the extent of spalling and cracking of the
parent metal increases. The difference in
performance of armor of two degrees of
toughness will be the greatest at lower tem¬
peratures. Weight and velocity of spall frag¬
ments increase with increasing brittleness of
rolled homogeneous armor
6. The spalling and cracking of rolled
homogeneous armor increases as the tempera¬
ture decreases.
3-9 OTHER TYPES OF WARHEADS
Other types of warheads which find use in
recoilless rifle weapon systems are antiperson¬
nel (APERS), incendiary« white phosphorus
(WP), smoke, and chemical types.
The APERS or canister type warhead con¬
sists of a nonexplosive thin-walled shell
loaded with a large number of small pre¬
formed missiles. The projectile is designed in
such a manner that it breaks up under the
action of centrifugal forces as it leaves the
weapon muzzle, scattering the missiles in a
cone-shaped pattern in front of the rifle in
order to obtain a short-range lethal effect on
personnel.
The incendiary type warhead contains a
projectile filler that will produce a high
enough temperature to ignite any flammable
material in the target or incapacitate person¬
nel.
The WP type warhead is designed with a
projectile that is very similar to that of the
same size HE projectile. The projectile com
tains a filler of white phosphorus that
produces a white cloud when dispersed from
the projectile by a high explosive contained in
a metal burster tube in the center of the
projc tile.
The smoke type warhead is again similar in
configuration to the HE type projectile. The
projectile contains steel canisters filled with a
colored smoke composition that is ignited by
quick-match in a flash tube which is in turn
fired by a black powder initiator from the
fuze. When the projectile functions, an
ejection charge ejects the steel canisters and
the burning dye composition is spread on the
ground.
The chemical filled warheads are very
similar in design to the WP type except that
more rigid fits and tolerances are required to
seal against the premature leakage of the
contents. The design of the liquid-filled
projectile burster casing is similar to the
burster casing used in the WP projectile
except that it is slightly larger to prevent the
burster from whipping around inside the
projectile. Sufficient charge is provided to
open the projectile and disseminate the liquid,
which is cither in the form of a persistent or a
nonpersistent gas.
REFERENCES
1. AMCP 706-245(C), Engineering Design
Handbook, Ammunition Series , Section 2.
Design for Terminal Effects (U).
2. AMCP 706-107, Engineering Design Hand¬
book, Elements of Armament Engineering ,
Part Two, Ballistics.
3. AMCP 706-170 (S-NOFORN), Engineering
Design Handbook, Armor and Its Applica¬
tions (U).
4. AMCP 706-290 (C) Engineering Design
Handbook, Warheads-General (U).
AMC? 706-236
CHAPTER 4
EXTERIOR iW-USTICS
4-0 LIST OF SYMBOLS
A
= Siacci altitude function for
= static moment coefficient.
fixed-fin projectiles, dimen¬
A
rad 1
sionless
C M.
= magnus moment coefficient.
a o
= speed of sound in air at
rad' 1
normal atmosphere condi¬
tions = ! 120.27 fps
C M + Cu .
= damping moment coeffi¬
W A
cient, rad"*
C
= ballistic coefficient, lb-ini 2
c»
= aerodynamic force coeffi¬
Cq
= drag coefficient, dimension¬
cient associated with nor¬
less
mal force, dimensionless
Cp f
= wave drag coefficient, di¬
C *p
= aerodynamic force coeffi¬
J
mensionless
p
cient associated with mag¬
nus force, dimensionless
Cp Q
= drag coefficient at zero
o
yaw, dimensionless
Cn _
= magnus force coefficient.
pa
rad' 1
C p.
~ drag coefficient of a known
piojectile design, dimen¬
C N a
= normal force coefficient.
sionless
rad -1
dC D
c.p.
= center of pressure
Cp.
* —— rate of change of
diet 1 )
CP
= distance from projectile
Cp with a 2 , ra<T 2
base to center of pressure.
cal
CG
- distance from projectile
base to center of gravity, cal
D
= drag force, lb
c.g.
= center of gravity
d
= maximum body diameter of
projectile, ft
c L
= aerodynamic force coeffi¬
cient associated with lift
G(u)
= drag function Ib-finF-secT*
force, dimensionless
g
~ acceleration due to gravity.
C L a
* lift coefficient, rad"'
ft-sec -2
4-1
AMCF 706-238
H
h
I
i
K
k ,
L
M
My
m
N
N P
P
Pi
dimensionless
= equilibrium roll rate,
rad-sec " 1
= resonance roll rate,
rad-sec " 1
s altitude Siacci equation q
iri^-ft-lb " 2
, . Qi
- Siacci inclination function
for fixed-fin projectiles, di¬
mensionless R
= dynamic pressure, psf
= inclination, Siacci equation,
in?-lb " 1
= range, yd
- axial moment of inertia,
slug-ft 1
= retardation of projectile,
sec " 1
* transverse moment of iner- 5
tia, slug-ft 2
S
- 1) 1 y 2 ; in complex nota¬
tion indicates rotation by
90 deg
s,
= form factor, dimensionless
= projectile frontal area, ft 2
= Siacci space function for
fixed-fin projectiles, dimen¬
sionless
= dynamic stability factor,
dimensionless
■ axial radius of gyration, cal
= transverse radius of gyra¬
tion, cal s
= lift force, lb
= Mach number, dimension- j
less
= dynamic stability factor for
\ max < 0 , dimensionless
= gyroscopic stability factor,
dimensionless
pSd
2m
Cl,
~ total moment about a hori- T j
zontal axis through projec¬
tile c.g., ft-lb
= Siacci time function func¬
tion for fixed-fin projec¬
tiles. dimensionless
projectile mass, slug t
normal force, lb t x
m&gnus force, lb
. '/
roll (spin) rate, rad-sec
U
space Siacci equation,
in?-ft-lb ** 1
= time, sec
= Siacci time equation,
in? -sec-lb ** 1
= time of flight, sec
= upper limit of integration
for a specific projectile
type, Siacci equation, fps
4-2
✓
AM CP 706-238
= projectile weight, lb
* displacement along x-axis,
ft
= velocity along x-axis, fps
d 2 x ,
= —, ft-sec' 2
“ displacement along z-axis, ft
= velocity along z-axis, fps
d 2 z
- 7 , ft-$ec' J
dt 2
= angle of yaw, vertical com¬
ponent, rad
= angle between horizontal
(x-axis) and velocity vector,
rad
= air density, slug-ft " 3
= air density, lb-ft " 3
- angle of elevation, mil
= angular velocity about hori¬
zontal axis when d = 0
SUBSCRIPTS
= yawing velocity about hori¬
zontal axis, rad-sec ' 1
= angle of yaw, horizontal
component, rad
= PoV’/W'. ff 1
- dummy index (to be re¬
placed by a sequence of
specific indices when the
subscripted quantity is to
be used in a computation)
* muzzle condition
= total angle of yaw, rad
= initial yawing velocity, rad-
= initial condition or zero
yaw value
- tail or terminal
= argument of Siacci func¬
tions: component along the
line of departure of the
velocity relative to air, fps
= projectile speed relative to
an inertial coordinate sys¬
tem, fps
= muzzle velocity, fps
= terminal velocity, fps
horizontal component of
velocity, fps
“ yaw angle of repose, rad
= half angle of nose cone, deg
= nutational damping expo¬
nent, dimensionless
~ precessional damping expo¬
nent, dimensionless
= static moment factor,
(ft-lb>rad-‘
aerodynamic jump angle,
rad
AMCP 706-238
SECTION I
INTRODUCTION
4-1 SCOPE
Exterior ballistics describes the motion of
the projectile from muzzle exit to point of
impact. The complete theory of exterior
ballistics includes only those effects that are
of primary interest in the design of recoilless
rifle ammunition, i.e., rounds normally of
caliber 57 to 120 mm in size that are
launched at muzzle velocities up to approxi¬
mately 2000 fps.
There are two major considerations in the
exterior ballistic design of an accurate
projectile: (1) the projectile must be stable in
flight, i.e., the projectile must be designed to
prevent tumbling and limit yaw to small
angles, and (2) given the initial conditions, the
trajectory of the projectile must be deter¬
mined.
These two considerations, stability and
trajectory calculations, comprise the major
portion of this chapter. These subjects will be
supplemented by a discussion of aerodynamic
coefficients and other basic material.
4-2 WEAPON SYSTEM INTERACTION
Exterior ballistic factors directly influence
the accuracy of the weapon system. To
illustrate, the accuracy of a conceptual
projectile having a perfectly flat trajectory
and zero time of flight is limited only by the
accuracy of the sighting device. However, as
tire time of flight increases, crosswinds and
other meteorological effects interact signifi¬
cantly with the projectile; and, further, as the
trajectory is elevated, range estimation errors
are introduced. In order to minimize these
errors, the exterior baliistician is concerned
with the projectile weight and mass distribu¬
tion, shape, end muzzle velocity.
4-3 QUALITATIVE DESCRIPTION
The final result of exterior ballistic
calculations is a trajectory describing the
position of the projectile center of mass as a
function of time when fired with a given
muzzle velocity and superelevation angle-the
angle between the gun axis and the line of
sight to ,the target. Calculation of the
trajectory is a routine computer operation,
provided projectile drag is known. The
FORTRAN particle trajectory program pre¬
sented in Ref. 1 is an example of such a
computer program. However, before making
trajectory calculations, the projectile must be
stabilized to assure that it will not tumble or
yaw excessively during flight. There are two
methods of aerodynamic stabilization: (1)
gyroscopic stabilization, i.e., spinning the
projectile, and (2) fin stabilization. The mass
distribution determining the location of the
center of gravity and the shape determining
the location of the center of pressure are
critical in both of these methods.
'The theory is well established and stability
and trajectory calculations can be made,
provided the forces acting on the projectile
are known. These forces are expressed in
te ms of aerodynamic coefficients which are
discussed in Section IL
In brief, the projectile is stabilized by
adjustment of the mass distribution (location
of center of gravity), by adjustment of the
external shape (location of center of pres¬
sure), and in some cases, by the spin rate of
the projectile. The designer then minimizes
4-5
Preceding page blank
AMCP 706-238
drag to obtain the ‘‘flattest” and shortest
time-of-fiight trajectory. Optimization also
involves maximizing both the muzzle velocity
and sectional density (mass per unit cross-sec¬
tional area) of the projectile, subject to
constraints on the overall cartridge weight,
recoil momentum, peak pressure, cartridge
profile, and charge-to-mass ratio.
4-6
AM CP 706-238
SECTION II
AERODYNAMIC FORCES AND MOMENTS
44 GENERAL
The aerodynamic forces on a projectile are
determined by the pressure distribution
existing over the entire projectile exterior. In
order to simplify their measurement and
mathematical use, the distributed aerody¬
namic forces are grouped into a specified set
of resultant forces. The set of (resultant)
forces and moments which have a significant
effect on the projectile motion is composed
of
1. Normal force
2. Lift
3. Drag
4. Magnus force
5. Static moment
6. Damping moment
7. Magnus moment
8. Roll damping moment.
4-5 AERODYNAMIC FORCES
45.1 NORMAL, LIFT, AND DRAG
FORCES
The resultant of the pressure forces on a
symmetrical nonspinning projectile lies in the
plane containing the tangent to the trajectory
and the longitudinal axis of the projectile,
called the “yaw plane”; the point on the
projectile axis through which this resultant
passes is called the center of pressure of the
lift or normal force, since the resultant may
be resolved either into lift and drag
components, or into normal force and axial
drag. Lift is parallel to the y, z-plane, drag is
parallel to the x-axis; normal force is
perpendicular to, and axial drag is in line
with, the axis of the projectile. Each possible
pair of components lies, of course, in the yaw
plane (Ref. 1). Definition of axis is as given in
Fig. 41.
45.2 MAGNUS FORCE
When a projectile is spinning about its
longitudinal axis, the pressure distribution
over its surface is altered so that the resultant
force no longer lies in the plane of yaw. This
is resolved by introducing a force component
normal to the yaw plane, together with its
associated moment. This force, called the
“magnus force”, is also perpendicular to the
longitudinal axis of the projectile, and passes
through its own center of pressure. Vector'
subtraction of the magnus force from the
total force on the projectile leaves a force in
the yaw plane, which can be resolved into lift
and drag (Ref. 1).
46 AERODYNAMIC MOMENTS
46.1 STATIC MOMENT
The static moment is the product of the
normal force and the distance between its c.p.
and the c.g. of the projectile, which is
considered positive when the c.p. is forward
of the c.g. as it practically always is for
47
AMCP 706-238
Z
Figure 4-1. Coordinate System
spin-stabilized projectiles. The axis of this
moment is a transverse axis through the c.g.,
normal to the yaw plane. Fin-stabilized projec¬
tiles have the c.p. aft of the c.g., so that the
static moment opposes an increase in yaw (in
normal flight), and can be called a “restoring
moment” (Ref. 1).
4-6.2 DAMPING MOMENT
Yaw varies continuously throughout the
projectile flight, and, as this angle is changing,
the projectile swings about its c.g. This action
changes the pressure distribution on the
projectile so as to produce a couple about an
axis through the c.g. normal to the plane of
the yawing velocity (which is not necessarily
the plane of yaw). This couple is called the
“damping moment” and usually opposes the
yawing velocity (Ref. 1).
4 6.3 MAG^ 3 MOMENT
The magnus force produces a moment
about an axis through the c.g., parallel to the
normal force. This magnus moment changes
the yawing velocity in a manner depending on
the location of the c.p. of the magnus force
and its direction. Because the magnus force
and moment result from the projectile spin,
they are absent on a nonrotating projectile.
However, the complete absence of magnus
effects on fin-stabilized projectiles generally
cannot be stated since fin-stabilized projec¬
tiles often are given a slow stabilizing spin.
4-6.4 ROLL DAMPING MOMENT
As defined in Ref. 1, the roll damping
moment is a couple about the longitudinal
axis of the projectile and, for a spinning body,
is related to the friction between projectile
and air. Fins produce larger roll damping
moments owing to the angle of attack
produced by the spin.
4-8
AMCP 706-236
4-7 FORCE AND MOMENT COEFFICIENTS
Aerodynamic forces and the static moment
have been found to be proportional to the
projectile dimensions, to the dynamic pres¬
sure of the air, and to the projectile yaw. In
addition, the three moments arising from
projectile rotation are also proportional to
their appropriate angular velocities. The
factors of proportionality wlrich relate these
quantities are known as “aerodynamic coeffi¬
cients”. These coefficients are not constant
for a given projectile, but are functions of
Mach number, Reynolds number, spin rate,
and yaw as described in pars. 4-7.1 and 4-7.2.
4-7.1 AERODYNAMIC FORCE COEFFI¬
CIENTS
The most significant of the aerodynamic
force coefficients arc defined as follows:
Cjf -N/iqS) (4-1)
C L = L/(qS) (4-2)
Cq — D/ (qS) (4-3)
where
q - dynamic pressure, p V 1 12, psf
(4—5)
S = nd 2 /4, frontal area of the projec¬
tile (4-6)
N = normal force, lb
L = lift, lb
D - drag, lb
N ? = magnus force, lb
p = air density, slug-ft -3
V = speed of projectile relative to air,
fps
p - roll rate, rad-sec -1
d - maximum body diameter of
projectile, ft
The coefficients defined in Eqs. 4-1
through 4-4 are expected to be functions of
the yaw angle a, measured in radians. For
small yaw angles (a < 0.17 rad), all of the
aerodynamic force coefficients can be as¬
sumed to vary linearly with yaw. This
assumption leads to the use of a curve of
coefficient vs yaw angle as a more convenient
description of the characteristics of the
projectile. Eqs. 4-1, 4-2, and 4-4 then can be
written in the following form:
Cn = aerodynamic force coefficient
associated with the normal force,
dimensionless
C L - aerodynamic force coefficient
associated with the lift force,
dimensionless
C fi = drag coefficient, dimensionless
C Nn - aerodynamic force coefficient
p associated with magnus force,
dimensionless
N = {^) qSa " C W Sa ' lb < 4 ~ 7)
„ AfcA
L - - C L ^qSa, lb (4-8)
(4-9)
AMCP 706*238
where
dC N
- normal force coeffi¬
” ft
dot
dC,
cient, rad -1
C L*
_ L
doc
= lift coefficient, rad" 1
C N
"pa
= dC Np
= magnus force coeffi¬
d<x
cient, rad -1
a
= yaw angle, rad
For the sake of simplicity, the symbol a has
been used for the yaw angle. As indicated in
the notation of Ref. 1, a is the component of
the yaw angle in the vertical direction; the
component in the horizontal direction given
as 0, and the total yaw angle 6 given by
fi=j8+*a (4-10)
where i * y/~l, the unit vector in the complex
plane. Orientation of the yaw is then
Tan’ 1 (c'//3). The aerodynamic coefficients can
be defined in terms of a because of the
rotational symmetry of the projectile, and
their values derived from measurements made
on a model that is given a yaw in one place,
identified as the a-plane.
As indicated earlier in this paragraph, the
drag coefficient C D does not vary linearly
with yaw. It has been found that drag D varies
with the square of the yaw, so that
D = (C Do +C Da2 a 2 ) qS (4-11)
where
C D - drag coefficient at zero yaw,
dimensionless
C D 2 = rate of change of C D with a 2 ,
a rad' 2
4-7.2 MOMENT COEFFICIENTS AND MO¬
MENTS
The moments produced by the aerody¬
namic forces are referred to the center of
gravity of the projectile except as indicated
herein. In the terminology of this handbook,
the moment coefficients are derivatives with
respect to yaw, or with respect to the
appropriate angular velocities. The moment
coefficients of primary importance are:
1. Static Moment Coefficient C M a
c "« n “ r ‘ <*-w>
2. Damping Moment Coefficient
rad" 1 (4-13)
where
a = yawing velocity about the horizon¬
tal axis, rad-sec n
os = angular velocity about the horizon¬
tal axis when it = 0; i.e., the total
angular velocity about the horizon¬
tal axis is ce + ci, rad sec -1
jM y - total moment about a horizontal
axis through the c.g., ft-lb
3. Magnus Moment Coefficient C„
pa
dCy. ,
C ^ = “5^ >rad " (4 ~ 14)
In coefficient form, the total moment M y
about a horizontal axis through the projectile
is given by
4-10
AM CP 706-238
4-8 DETERMINATION OF AERODYNAMIC
COEFFICIENTS
The aerodynamic coefficients can be
measured by ballistic range testing or wind
tunnel testing. In the typical situation, the
recoilless rifle designer is interested in
estimating the value of the coeffiients for
preliminary system design purposes. These
preliminary estimates can be verified and
adjusted by actual measurements at a later
stage in the system development. The
methods of estimating the aerodynamic
coefficients are based on interpolation of
measurements on existing projectiles and on
theoretical calculations. For methods of
estimating the various aerodynamic coeffi¬
cients, the reader is referred to the material
found in Ref. 1.
4-11
AM CP 706-238
SECTION III
PROJECTILE STABILITY
4-9 INTRODUCTION
Projectile stability relates to the ability of
the projectile to quickly reduce the initial
yaw to a small value and thus minimize drag
and drift. Several stability criteria must be
considered. If the projectile is neither
statically nor gyroscopically stable, it will
tumble immediately after muzzle exit and be
inaccurate.
If it is dynamically unstable, the initial yaw
will increase with time and the projectile will
eventually tumble. In this section, equations
are presented for evaluating the various
stability criteria of a proposed projectile
design.
The material in this section is presented in
greater detail in Ref. 1.
4-10 BASIC STABILITY CONSIDERATIONS
As stated in Section II, the aerodynamic
forces acting on a projectile can be grouped
into a specific set of resultants. These
resultants have both magnitude and direction,
and also a point of application on the body,
i.e., a point through which the resultant acts.
This point, called the center of pressure c.p.
of the force in question, is assumed to lie in
the longitudinal axis of the projectile, while
its position along the longitudinal axis
depends on the shape of the projectile, the
projectile airspeed (Mach number), axial spin
rate, and, unfortunately, sometimes on the
magnitude of the yaw (Ref. 1).
The c.p. of the lift forces is assumed to be
independent of the yaw angle. This assump¬
tion is made possible by considering only
linear projectile behavior in which yaw
seldom exceeds 10 deg. Since the p 1 pcsc of a
good design is to keep the , aw Mow S deg,
the assumption of linear projectile behavior is
validated further when this design criterion is
achieved. However, the c.p. of the magnus
forces will exhibit appreciable movement
when the yaw angle changes as much as 10
deg.
The position of the c.p. relative to the
projectile c.g. is an important measurement of
the projectile stability. A projectile will be
statically stable if the c.p. is aft of the c.g.,
i.e., any yaw of the projectile produces a
moment about the c.g. which tends to return
the axis of 'he projectile to the zero-yaw
position. If the c.p. is ahead of the c.g., the
normal force produces an overturning mo¬
ment tending to increase the yaw, and the
projectile is said to be statically unstable. The
statically unstable projectile can be stabilized
by either spinning or adding fins to the
projectile. By spinning the projectile rapidly
about its own axis, tire yaw will not grow and
the projectile is said to be gyroscopically
stable, even though it is still statically
unstable. Addition of fins to the rear part of
the projectile body moves the c.p. rearward of
the c.g. and the fin-stabilized projectile
becomes statically stable. Further aspects of
the fin- vs spin-stabilization consideration are
described in pars. 4-11 and 4-12.
4-11 SPIN STABILIZATION
4-11.1 GYROSCOPIC STABILITY
If the projectile is given sufficient spin, the
4-13
Preceding page blank
AMC? 706-238
yaw angle will be small even though the
projectile is statically unstable. This is
analogous to the spinning top which remains
upright only when the spin rate is sufficiently
large. The condition for stability is expressed
in terms of the gyroscopic stability factor s g
as follows:
s t -
IW
dimensionless
where
(4-16)
I x - axial moment of inertia, slug-ft 2
I y - transverse moment of inertia,
slug-ft 2
p - axial angular velocity, rad-sec" 1
p = static moment factor, lb-ft-rad" 1
Assuming that the static moment p varies
linearly with yaw, one obtains:
p = ^pd s V 2 Cy , (ft-lb)-rad" 1 (4-17)
O Q
only on the rifling twist, i.e., the spin rate p
increases with muzzle velocity and p/V is
constant for a given twist. Thus, to a first
approximation, gyroscopic stability is inde¬
pendent of muzzle velocity and depends only
on rifling twist at the muzzle.
4-11.2 YAW OF REPOSE
During the flight of a spin-stabilized
projectile, the angle between the tangent to
the trajectory and the direction of the
longitudinal axis of the projectile quiets down
to a nearly constant yaw, called the yaw of
repose. The equilibrium condition is gener¬
ated when the gravity curvature of the
trajectory gives rise to an angle of yaw large
enough to create a precession rate that
permits the projectile axis to follow the
tangent to the trajectory. If the projectile spin
is clockwise when viewed from the rear, the
equilibrium requirement causes the projectile
to point to the right of its flight path
(right-hand yaw of repose). This yaw angle
generates a lift force that causes the projectile
to drift to the right.
where
p = air density, slug-ft" 3
d = maximum body diameter, ft
V - airspeed, fps
An approximate expression for the right
hand yaw of repose 6 r is
» _ l *Pg cos 0 I*, . . / c u M \pd']
where
'(4-18)
Cju a = static moment coefficient, rad" 1
If s g is less than unity, the projectile will
tumble within a few hundred feet of the
muzzle. If s g is greater than one, the projectile
is gyrcrcopically stable and dynamic stability
must be investigated as described in par.
4-11.3. To allow for variations in air density
and other factors, a value of s g of about 1.3
usually is desired for the preliminary design
stages. However, since most projectiles lose
airspeed faster than spin rate, the val^e of s g
increases with flight time. Note that s g
depends on the ratio (p/K) 2 and p/V depends
5 - acceleration due to gravity, ft-sec -2
6 = angle between the horizontal and
velocity vector, rad
An analysis of the first (and most
significant) term on the right side of Eq. 4-18
helps to explain the mechanism by which a
spinning projectile “trails” as it moves along
its trajectory. Multiplying both sides of Eq.
4-18 by pSdC M V 3 /2 gives
a
tpV 2 SdCy a 6 r = I zP +.)
(4-19)
4-14
AMCP 708-238
The left side of Eq. 4-19 is the static
aerodynamic moment. The first term of the
right side of Eq. 4-19 is the product of the
axial angular momentum I x p and the rate of
change of direction of the tangent to the
trajectory (g cos 0)1 V. As explained in Ref. 1,
Eq. 4-19 now states that the aerodynamic
moment arising from the yaw of repose is just
sufficient to change the angular momentum
of the projectile at the rate required for the
axis of the projectile to remain tangent to the
trajectory (in the vertical plane the yaw is in a
plane normal to the trajectory plane and the
static moment is at right angles to the
rotation, or precession, of the projectile axis,
which is the well known gyroscopic behavior).
4-11.3 DYNAMIC STABILITY
A spinning projectile has a gyroscopic
motion similar to a spinning top. The spin
axis of the projectile has a processional and
nutational damping exponent X, and a
processional damping exponent X 2 . If the
associated exponent X* (/ = 1,2) is positive,
the amplitude of motion increases with time
and the projectile eventually will tumble even
though it is statically stable. If the associated
X; is negative, the motion is damped and
reduces to zero. For dynamic stability both
\i and X 3 must be equal to or less than zero.
The analysis of dynamic instability is
particularly difficult since, for example, a
small dynamic instability might be tolerated
for a short time of flight. A complete analysis
of dynamic instability requires a sophisticated
computer program. However, for preliminary
design purposes, a dynamic stability factor s d
can be examined. In Ref. 1 the damping
exponents are defined as follows:
pSd r a -)
T - 2 -K c **J' dl »«“stonless
k t = transverse radius of gyration,
= , dimension! esu
k a = axial radius of gyration,
= U x /intd l )] i/2 , dimensionless
m - projectile mass, slug
Instead of simply requiring that X t and X 2
be nonpositive for stability, it is possible to
set an upper limit on the greater of the two
exponents which must not be exceeded if the
projectile is to remain stable- This limit,
represented by an unsubscripted X, may be
greater than zero because some growth of
initial yaw may be tolerable, especially in
short fligits (Ref. 1).
With the use of X, the dynamic stability
factory can be defined (Ref. 1) by
dimensionless (4-22)
In practice X *s often set equal to zero, and
the stability factor for \ max < 0, s d
becomes °
Sd
_ 2 T _ 2 (c La +
H ~ C La - Co- k?(C u + C„. a )
(4-23)
In Ref. 1, it is shown that stable values of s d
are related to values of the gyroscopic
stability factor s g as:
— = s d (2 - s d ) (4-24)
Sg
dimensionless (4-20)
This relation is plotted in Fig. 4-2 which
shows the span of acceptable values of s d as a
function of l/s g .
X 2 - - Xi, dimensionless
where
(4-21) 4-H.4 AERODYNAMIC JUMP OF SPIN-
STABILIZED PROJECTILES
H = ■ c “~ k ‘ 2 ( Cl, » *
dimensionless
Ideally, the path of the projectile at muzzle
exit coincides with the bore-sight of the gun.
In practice, the projectile path deviates from
4-15
Inverse of Gyroscopic Stability Factor
Slow Mode
Less Stable
Fast Mode
Less Stable
Gyroscopic 1
Instability (7 >1)
1 .0 Magnus
[ Instability
(s d < 0 )
Region of Dynamic Instability
Weak Damping
Moment (s d >2)
D.4 _ Static
Instability
0.2- <i>«
Gyroscopic
Stability (7 < 1)
3 .2Static
Stability
I < 0 )
s g
_Region of__
Dynamic Stability
= s d ( 2 -s (
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2l0
Dynamic Stability Factor s
Figure 4-2. Graph of 1/s fl vs s d (Ref. 1)
r
706-238
AMCP 706*238
the gun bore-sight due to gravity, wind, drift,
and aerodynamic jump. Aerodynamic jump is
defined as the deviation angle that remains
after the effects of gravity, wind, and drift
have been eliminated. It includes such effects
as balloting, poor obturation, motion of the
gun tube, projectile asymmetries, and the
effects of muzzle blast. The angle of jump
6j— to a close approximation-is given in
complex form by
Oj - ^ 2 *. <*& - iilP'dJ , rad (4-25)
where
c* a - C HaB (CP B ~ CG) + C Har (CP T - CG)
(4-27)
CP
(4-28)
NOTE: No subscript refers to the overall
projectile; subscript T refers to the
tail; subscript Q refers to body
alone; and the distances from the
projectile base to the center of
pressure and center of gravity, CP
and CG , respectively, are measured
in calibers.
V Q - projectile velocity, fps
6 0 = yawing velocity, rad sec -1
p Q » spin rate, rad-sec’ 1
measured
at the
end of
the blast
zone
6 o = yaw, rad
Note that Eq. 4-25 describes two compo¬
nents of the jump angle £;.
4-12 FIN STABILIZATION
11Z1 INTRODUCTION
For a statically stable projectile, CP - CG
is negative, but this quantity usually is treated
by its absolute value and referred to as the
“CP - CG separation” in units of calibers.
Although an optimum magnitude of the CP -
CG separation is not well-defined, certain
limits have been established. The CP - CG
separation should be far enough above 0.5 cal
so that any inaccuracies in estimating Cm q
and C N will not cause the CP - CO'
separation to fall below 0.5 cal. On the other
hand, CP - CG separations above 1.0 cal have
been found to increase the dispersion at the
target (Ref. 1).
The usual projectile body is not statically
stable, i.e., the center of pressure is forward
of the center of gravity of the body. By
addition of fms rearward of the c.g., the c.p.
is moved rearward of the c.g. and the
projectile is made statically stable. If. when
the fin-stabilized projectile is yawed, the
moment produced by the lift forces acting on
the fins is greater than the moment produced
by the forces acting on the body, the net
moment will oppose the yaw and the
projectile will be statically stable. In symbolic
notation, the stability of a fin-stabilized
projectile is defined in terms of aerodynamic
moment coefficients as follqws:
4-12.2 FIN TYPES
The choice of a specific fin type is a
trade-off problem involving the utilities of
projectile volume, range, accuracy, and cost.
The effects of those design criteria on the fin
type selection and the overall effec? of
fin-stabilization on ooturation are described
briefly in this paragraph.
Fixed fins of one caliber span are easy to
make with a high degree of uniformity: this
promotes accuracy. However separation is
required between the leading edge of the fin
and the location of the full body diameter of
the projectile in order to permit the air stream
to expand and flow over the fin surfaces to
develop the expected lift.
AMC? 704238
Folding fins are bunched behind the
projectile when in the gun tube and then fan
out to a more than one caliber span after the
projectile has left the muzzle blast. Folding
fins can produce large CP - CG separations,
but are expensive and conducive to large
projectile asymmetry. However, they do not
reduce the projectile volume-to-Sength ratio
(an important drag consideration) as much as
fixed fins.
While fin-stabilization of either type results
in a larger and more expensive projectile, the
reasons for using fin- instead of spin-stabiliza¬
tion are
1. Shaped charge penetration is degraded
by spin.
2. The internal structure of the projectile
may be such that it cannot be made
gyroscopically stable.
3. Projectile is fired from a smooth bore
gun.
4-12.3 DYNAMIC STABILITY
As discussed in par. 410, a projectile is said
to be dynamically stable if its transient yaw
does not increase during flight. Statically
stable fin-stabilized projectiles having zero
spin are always dynamically stable. However,
a condition of zero spin almost never exists
since manufacturing tolerances permit some
slighj twist of the fins that results in a spin
producing torque. In fact, zero spin is
undesirable, because the lift produced by the
projectile asymmetry will steer the projectile
away from its predicted trajectory.
The effects of the projectile asymmetry on
the trajectory can be minimized by giving the
projectile a slow roll caller! a slow spin. The
desired slow roll is much smaller than the spin
rates given to spin-stabilized projectiles, and
often is produced by “canting” the fins.
Computation of this equilibrium roll rate is
described in detail in Ref. 1. It is important to
have a good estimate of the equilibrium spin
rate. In other words, the spin rate should be
kept low enough to avoid magnus effects as
described in par. 412.5.
412.4 AERODYNAMIC JUMP OF FIN-STA¬
BILIZED PROJECTILES
The material presented on aerodynamic
jump in par. 411.4 applies without change to
fin-stabilized ammunition except that the
drift of a fin-stabilized projectile is kept small
by rolling the projectile slowly. However, a
very good design is required in order for the
fin-stabilized projectile to achieve the same
low level of aerodynamic jump as that of a
spin-stabilized projectile fired in the same
gun.
Since CP - CG * C M JC L for small
angles of yaw, it is seen from Efq. 4-25 that
the aerodynamic jump angle d, can be
reduced by increasing the CP - CG
separation. If the increase in the CP - CG
separation is achieved by increasing the
moment coefficient of the tail (by greater fin
area or a longer boom), the initial yawing
velocity is also increased due to the
increased effectiveness of the fins in the
reversed flow of the blast zone. If the increase
in y a is greater than the increase in CP - CG
separation, the aerodynamic jump will be
increased and not decreased by the change of
CP - CG seperation.
By changing the shape of projectile body, it
is possible to move the c.p. of the normal
force rearward. This change can increase the
CP - CG separation of the whole projectile
with little or no change in the tail moment.
However, if this body change is made by
substituting, a spike for the ogive, the drag will
be increased.
It is noted again that aerodynamic jump
has been discussed only for dynamically
stable projectiles where 5 0 arid the CP - CG
separation are of primary interest. Fin-stabi-
AM CP 706-238
lized projectiles that are statically stable are
also dynamically stable unless they have an
unusually high roll rate (Ref. 1).
4-12.5 MAGNUS STABILITY
As stated in Ref. 1, an unbalanced side
force can be created on a slowly rolling,
fin-stabilized projectile when the projectile is
yawed and the body blankets the leeward
fins. This force, identified as a magnus force,
and its associated moment can be as large as
the magnus force and moment acting on a
spin-stabilized projectile. If the fins are
canted, the fin lift, upon reaching an
equilibrium spin, acts in opposite directions
on the in-board and outboard fins sections,
leading to a nonlinear magnus moment with
yaw.
In any case, the magnus moment coeffi¬
cients of fin-stabilized projectiles are less
predictable than those of spin-stabilized
projectiles. Thus, it is wise to allow as great a
margin of dynamic stability as possible
without falling into resonance instability,
which is discussed ir. par. 4-12.6.
4-12.6 RESONANCE INSTABILITY
If either the nutational frequency or
processional frequency r> nearly equal to the
projectile spin frequency, the magnitude of
the yaw due to any projectile asymmetry can
become very large. The similarity of this
phenomenon to a spring-mass system subject¬
ed to an external alternating force has led to
the use of the term “resonance instability” as
a label for this type of yaw increase. The
increase in yaw, unlike the growth of the
amplitude of an ordinary spring-mass system,
is bounded not so much by the damping in
the system as by its nonlinearity. The
resonant yaw of a projectile may become
large enough to cause loss of range and
accuracy through large drag increases, but not
so large as to cause the projectile to tumble.
While both spin- and fin-stabilized projectiles
theoretically can experience coincidence of
spin and yaw frequencies, this phenomenon is
much more likely to occur in fin-stabilized
projectiles.
In Ref. 1, it is shown that the spin rate
which results in this objectionally large yaw
angle, identified as the resonance roll rate p r ,
can be estimated by
where
p = qSdC M , static moment factor,
(ft-lbj-rad -1
Eq. 4-29 indicates that when l y is greater than
I x , which is always the case, p must be
negative for resonance, i.e., only statically
stable ( < 0) projectiles can exhibit
resonance instability.
The equilibrium roll rate p e must be
designed to avoid the resonance value p r . If
the projectile has zero spin at muzzle exit, the
value of p t should be made sufficiently high
so that the spin rate will quickly pass through
the value p t . A ratio of three for p e \p, is
considered desirable. It also is recommended
that the obturator (see Chapter 11, par.
11-12) be designed to produce a spin rate at
the muzzle exit greater than p r and thus avoid
passing through the resonance spin rate in
flight. This avoids the phenomenon of “roll
lock-in” in which the spin rate “locks in” at
the resonant frequency rather than increasing
to the designed equilibrium spin rate.
AMCP 706-238
SECTION IV
AERODYNAMIC DRAG
4-13 GENERAL
The drag force D was given by Eq. 4-3 as
D=qC D S, lb (4-30)
where C D is the drag coefficient which from
Eq. 4-11 is a function of yaw angle, i.e.,
C D = 0 Qq + C^2 (4—31)
The material on aerodynamic drag con¬
tained in Section IV is confined to the drag of
a projectile flying at zero yaw. The drag
coefficient at zero yaw C Do , in this situation,
can be called the axial drag coefficient. For a
well behaved projectile, the initial yaw damps
rapidly to a small value, so that by far the
most important term of C D in Eq. 4-31 is
Cd 0 . The minimization of C D<> consistent
wifii internal volume requirements, is, mere-
fore, of primary importance in the projectile
design. A decrease in C D permits the same
accuracy with a lower muzzle velocity and a
resultant reduction in weapon system weight.
The axial drag at zero yaw can be divided
into three components:
1. Wave Drag: the component associated
with the formation of shock waves.
2. Friction Drag: the drag due to the
flow of air over the projectile body.
3. Base Drag: the component resulting
from the reduced air pressure on the base of
the projectile.
Each of the drag components depends
strikingly on the Mach number region, which
for recoilless weapon projectiles is broken
down into the following approximate ranges
that will vary slightly according to the specific
projectile shape:
1. Subsonic Range: M <0.8
2. Transonic Range: 0.8 <M < 1.1
3. Supersonic Range: 1 <M < 5
It is not practical to determine the drag
coefficient as a function of Mach number for
each projectile design. Instead, the drag
coefficient of a projectile shape that differs
slightly from a typical projectile design is
assumed to be proportional to the drag
coefficient of the typical projectile (Ref. 2).
If Co, is the drag coefficient of a known
projectile design, the form factor i t of a
projectile whose drag coefficient is C D is
given by
it~C D /C Dt , dimensionless (4-32)
The ballistic coefficient C of the projectile
relative to the known drag coefficient is
defined as
C = W/(144t t rf 2 ), lb-inT 2 (4-33)
where
W = projectile weight, lb
d = maximum projectile diameter, ft
Values of the form factor i, and b; llistic
coefficient C, are given in Table 4-1 for
existing recoilless ammunition.
4-21
Preceding page blank
ig*im i^m m if.
AM CP 700-238
TABLE 4-1
RECOILLESS AMMUNITION CHARACTERISTICS (Rtf. 11)
Projaetilt Effactivt
Oiamatar, Rang*, '
Rffla
in.
yd
Ammunition
67 mm Ml8
2.244
500
M306A1 HE
M307 HEAT
76 mm M20
2.953
1000
M309HE
M310 HEAT
90 mm M67
3.543
500
M371 HEAT
106 mm M27
4.134
1000
M323 HE
M324 HEAT
106 mm M40
4.173
1200
M344 HEAT
ProiactUa
Wilght,
lb
Initial
Vaiocity.
fp«
Ballittie
Cotffi-
dint
C
Form
Factor
/
Drag*
Func¬
tion
G
2.75
1200
0.608
0.90
1
2.74
1200
0.52
1.05
1
14.40
990
1.473
1.12
5
13.10
1000
1.781
0.84
6
7.0
800
0.429
1.3
1
32.4
1120
2.23
0.85
5
29.3
1250
1.34
1.28
5
17.6
1650
1
1
M344
*Sm par. 4-30.3.2
In terms of the known drag, and ballistic
coefficients (Eqs. 4-32 and 4-33), the
expression for the aerodynamic drag/) in Eq.
4-30 becomes
qWC Dt
4/ 144C
lb
(4-34)
4-14 SUBSONIC VELOCITIES
For subsonic Mach numbers, the drag
coefficient is roughly constant. In this region,
the “teardrop” is a good aerodynamic shape,
i.e., the projectile should have a rounded nose
and a small base diameter. Blunting the nose
of projectile (short of a completely flat face)
has little effect on overall drag in the subsonic
range, but has the important effect of
reducing the critical Mach number.
Base drag is the result of a pressure
deficiency over the projectile base more
commonly evident in the substatic (less than
atmospheric) pressure existing in the wake of
a train or automobile. Base drag is reduced by
“boattailing” or tapering the rear of the
projectile to reduce the base area. However,
only a limited amount of tapering can be
tolerated since boattailing reduces the lift
coefficient and moves the c.p. of the normal
force forward, thus reducing the stability of
the projectile. Large boattail angles (greater
than 16 deg) without a rounded transition
from the cylindrical body also can cause air
flow separation at the base-body junction,
canceling all of the drag reduction achieved
by boattailing.
Interruptions in the smooth contour of the
projectile may cause an increase in the
friction drag. By proper orientation or
elimination of such surface irregularities as
slots, shallow holes, or protuberances, it is
possible to minimize the friction drag.
4-22
AMCP706-23S
4*16 TRANSONIC
The transonic range is characterized by the
formation of shook waves and a sharp increase
in drag due to a rapidly increasing drag
coefficient. The greatest part of the drag
increase in the transonic range is attributed to
the presence of the shock waves and is called
the wave drag. Wave drag is affected by
abrupt changes in the projectile shape, such as
the rotating band and undercuts on the body,
because the local Mach number varies from
point to point along the projectile surface
depending upon the projectile shape. In the
transonic range, base drag peaks at M - 1.0
and friction drag increases and becomes
relatively small as C D increases. Since rapid
and sometimes unpredicted changes in the
aerodynamic coefficients can occur in the
transonic range, the designer attempts to
minimize the flight time in this region.
4-18 SUPERSONIC
After the shock wave system is fully
developed (between Mach 1.1 and 1.2) the
drag decreases and is largely determined by
the shape of the nose. By the Taylor-Macoll
equation:
C D/ = [0.0016 + (0.002/M 2 )]€ 1 ’ 7 ,
dimensionless (4-35)
where
C D = wave drag coefficient, dimension¬
less
e = half angle of nose cone, deg
M - Mach number
While this equation is useful for estimating
the effect of nose angle, the minimum drag
actually is obtained by using an ogive of large
radius.
Boattailing reduces the drag at supersonic
velocities provided the airflow does not
separate from the body. Thus, the boattail
angle usually is limited to about 8 deg and
one caliber in length. Beyond these critical
characteristics, the flow will separate from the
projectile forward of the base, resulting in a
C D that is greater than the minimum
attainable.
4-17 TYPICAL VALUES OF DRAG
Fig. 4-3 shows four projectile shapes which
have been adopted as standards for compari¬
son (Ref. 2). In Fig. 4-4, the value of the drag
coefficient C D is plotted vs Mach number for
the four projectile types. As seen in Fig. 4-4,
C D remains fairly constant in the subsonic
range, increases rapidly ir. the transonic range,
and then decreases in the supersonic range.
In general, it is assumed that projectiles
having the same shape and c.g. location will
have the same set of aerodynamic coefficients
when fired at the same Mach number. It then
is possible to make use of such data as
contained in Figs. 4-3 and 4-4 in evaluating a
new projectile design. However, there are a
few outstanding exceptions to this rule.
It has been found that stability can be
improved by replacing the ogive nose of the
projectile with a slender cylinder or “spike”
protruding from the flat forward face of the
body. During firing tests, it was found that
the spiked projectile exhibited essentially two
different drag coefficients that were deter¬
mined according to the position at which the
flow separated from the spike. This phenome¬
non was called dual flow. In order to avoid
the condition of dual flow, with its serious
effects on accuracy, a tripper ring is added
near the tip of the spike to insure the early
separation of the flow. Since the spiked-nose
projectile does increase drag, it is not
commonly used in rccoilless rifle projectiles.
4-23
AMCP 706-238
SECTION V
PARTICLE TRAJECTORY CALCULATIONS
4-18 TRAJECTORY PROBLEM
A detailed trajectory calculation uses six
coordinates to describe the motion of the
projectile. Three coordinates determine the
position of the projectile center of gravity,
two angular coordinates determine the projec¬
tile yaw angle, and a third angular coordinate
describes the roll or spin. This type of
trajectory calculation normally is made on a
digital computer (see Ref. 1) and will not be
considered in this discussion.
For the purpose of initial design calcula¬
tions of a proposed recoilless rifle weapon
system, a two-degree-of-freedom trajectory
calculation provides sufficient information.
Thus, the trajectory calculations to be
described in this section assume the projectile
yaw angle to be zero and the trajectory to lie
in the xz-piane. The deviation from the
xz-plane is a separate calculation based on the
effect of crosswind and drift for the
calculated time of flight.
The trajectory problem consists of integrat¬
ing the equations of motion of the projectile
given an initial velocity vector, the vertical
force of gravity, and on aerodynamic drag D,
acting in a direction opposite to the velocity
vector. (The determination of the numerical
value of D is performed according to Eq.
4-34.) For the typical short time-of-flight,
flat, recoilless weapon trajectory, the effects
of variation of air density with altitude,
Coriolis force, earth rotation, and similar
secondary effects can be neglected with little
loss of accuracy.
4-19 TRAJECTORY EQUATIONS
Fig. 4-S shows the coordinate system and
forces acting on the projectile for a
two-degree-of-freedom trajectory. Describing
the projectile motion in terms of x and z
components, the equations of motion are
mx = D cos 0
(4-36)
mi = D Bin 0 — mg
(4-37)
subject to the initial condition
tan 6 = i 0 /x 0
(4-38)
where
0 = angle between horizontal (x-axis) and
velocity vector, rad
A dot above the coordinate indicates a
derivadve with respect to time and subscript o
denotes initial (muzzle) value-
The equations of motion cannot be readily
integrated since D is a function of Mach
number M and 6 varies with time. The usual
procedure is to use a numerical integration in
which the drag D and flight angle 0 arc
considered constant for short intervals of
time.
4-20 SOLUTIONS OF THE EQUATIONS
Until the adoption of high speed digital
computers, trajectory calculations were per¬
formed exclusively by approximate methods
which employed average or effective values of
4-27
Preceding page blank
AMC? 706-238
z
Figure 4-5. Coordinate System for Trajectory Calculations
the drag coefficient. Examples of these
approximate methods are the simplified
exterior ballistics techniques described in par.
4-20.1 and the Siacci method outlined in par.
4-20.3. While the approximate methods are
still used for rapid estimations of the effects
of variations in projectile shape, muzzle
velocity, and quadrant elevation on range and
time of flight, complete trajectory calcula¬
tions are made on digital computers using
numerical integration techniques as described
in pa.*. 4-20.2. The use of digital computers
has resulted in accuracies of simulation better
than one percent, assuming that the drag
coefficient curve used averaged within 2
percent of the true C D at all Mach numbers
traversed.
4-20.1 SEMI EMPIRICAL EQUATIONS FOR
FLAT TRAJECTORIES
Calculations of the trajectory angle of
elevation d 0 , time of flight tj, terminal
velocity V t , and angle of descent 0 are
necessary for the ballistic evaluation of the
weapon system, but are very time consuming
because factors such as drag and ballistic
coefficients are not readily available. In Ref.
3, a simplified method (based on the
assumption of a flat trajectory) for calculating
these parameters is presented. From experi¬
mental and theoretical work which indicates
that the deceleration of a projectile is
approximately proportional to the square of
4-28
)
its velocity at any particular time,
shows that
Ref. 3
. _exp [(3 r m R)/V m \ - 1 _
h ~ • sec
T m
(4-39)
>/ ,
//, .
)
(4-40)
x) : V
$■.
(4-41)
jK
<t> = (1 +r m t f )0 o , mil
(4-42)
where
(dV\
r m ~ (- j = retardation at muzzle,
\d x jm sec' 1
R = range, yd
V m = muzzle velocity, fps
V x - horizontal component of velocity,
fps
and subscript m indicates muzzle condition.
The retardation and muzzle velocity are
obtained during charge establishment and
uniformity firings with the same system.
Eqs. 4-39 through 4-42 have proven to be
accurate within 3 percent for tf and Q 0 and
within 5 percent for V, in the experimental
ranges encountered in recoilless rifle systems.
In order to facilitate the use of the simplified
method, the formulas for tf, 6 0 , V t , and </>
have been incorporated into the nomogram
shown in Fig. 4-6.
The following sample operation for the use
of Fig. 4-6 is presented for the 106 mm, T170
system. Given the following data:
R = 1,000 yd
r m = 0.20 sec" 1
V m = 1,820 fps
1. Align muzzle velocity V m (1,820 fps)
with range R (1,000 ya) to determine point
on reference line 1.
2. Align the point on the reference line
1 with retardation r m (0.20 fps/ft) to
determine a point on reference line 2.
3. Align the point on the reference line
2 with muzzle velocity (already spotted) to
determine the desired angle of elevation 6 0
(18 mil).
Calculate the other parameters:
8 0 V*
16,100
18(1820)
16,100
= 2.03 sec
<#> = (1 + r m t f ) 9 0
= [1 + (0.2)(2.03)]18 = 25.3 mils
4-20.2 DIGITAL COMPUTER SOLUTIONS
A digital computer program is designed to
solve the trajectory equations in the same
manner as described in the numerical
integration method of par. 4-20.3. An
example of this type of program is found in
the FORTRAN particle trajectory program of
Ref. 1. A table of drag coefficients vs Mach
4-29
AMC? 704238
number is placed in the computer memory so
that the program can interpolate the value of
a drag coefficient between any two points
within the storou data deck. With this drag
information, the program can compute the
position and velocity of the projectile relative
to the coordinate system as well as pertinent
angles. With the inclusion of moment
coefficient data in the computer memory, it
would be possible for the program to verify
the gyroscopic and dynamic stability of the
projectile.
4-20.3 OTHER METHODS
4-20.3.1 Numerical Integration
Numerical integration of Eqs. 4-36 through
4-38 is performed in the following format for
use with an electronic calculator.
Given: V 0 ,d a , and D vs Mach number
M
Calculate: x 0 - V Q cos 6 0
= V 0 sin e a
From the table of drag data, determine D
for the given V 0 . Select an appropriate time
interval U, say 0.1 sec, depending on desired
accuracy. Calculate velocity components at
the end of first time increment as follows:
xi - x 0 - m) cos 0 O ] £U , fps (4-43)
Vi = y 0 — RD/'jm) sin B g +g]At , fps (4-44)
Calculate components of position at end of
first time increment:
xt = x B +x 0 &t — {{D/m) cos fl] At 2 /2, ft
(4-45)
z\ = z 0 + - [(D/m) sin 6 +g\£t l /2, ft
(4-46)
Note: D/m is negative when z is negative
Calculate new angle 0 S :
= Tan -1 (ii/Ki), rad (4-47)
Calculate new velocity:
Vl = (% + £\) U2 , fps (4-48)
Repeat process for succeeding /th incre¬
ments as follows:
ii - [(Dj.j/m) cos 6 {mi ]At [(4—49)
Z{ - X(.i — [(Djj/m) sin +#) Afj(4-50)
Xi - *i-i — [(Di-i/m) cos 0|.iJAf 2 /2
(4-51)
*< ~ *1-1 + *mA*
- [(D { .i/m) sin 6 imi +g]At 2 / 2(4-52)
B{ = Tan'^ij/ij)
(4-53)
V, = (x\+'z\) i/2
.(4-54)
4-20.3,: iacci Tables
In 1880, the Italian Col.
F. Siacci
introduced the following Space, Time, Alti¬
tude, and Inclination Functions to simplify
the calculation of trajectory data for the
types 1, 2, and 8 spin-stabilized projectiles
described in par. 4-17.
Space:
Pi =
■
(4-55)
Time:
h =
[ Zfe’ in,2 " 8e °" lb ’
(4-56)
5UK 2 1
t^ G i u )’ ^ n *
(4-57)
C u aidu
Altitude: h = I , in. 4 —ft—lb" 2
(4-58)
AMCP 70*238
Muzzle Reference
Angle of
Velocity, Range, Lines,
Retardation, Elevation
$
where
u = argument of Siacci functions: com¬
ponent along the line of departure
of the velocity relative to air (i.e., u
is related to tangential velocity V
by V = u cos 6 0 /cos 0), fps
G(u) = drag function, lbKinJ-sec)' 1
U = upper limit of integration for a
specific projectile type, fps
g = acceleration due to gravity, ft-sec -J
Values of C7(u) as a function of u have been
determined for the types 1, 2, and 8
projectiles and are contained in Refs. 4, 5,
and 6. Based on the applicable drag function
G(u), the Siacci functions have been tabulated
for the Air Force (Refs. 7, 8 and 9). Since
these Siacci Tables were arranged primarily
for use in computing aircraft firing tables,
they were often called Aircraft Tables.
Although the Aircraft Tables are not accurate
in computing complete trajectories, they can
be used to obtain trajectory data at
superelevations below 1S deg according to the
following formulas (see Ref. 2) when
variations in the air density and Mach number
are neglected.
x=C(p-p 0 ) cos 6 0 , ft (4-59)
4-31
AMCP7M-238
tf = C(i— tj, sec (4-60)
z -x tan 9 0 — C 2 (Jt — *„) + Cq, sec 9 a , fl
*#
(4-61)
tan 0 = tan G 0 - C(q i - q t ) sec 0 o (4-62)
and for z = 0
Note: Subscript o refers to initial value
and nonsubscripted qualities corre¬
spond to the desired value of u for
which the trajectory data is being
computed.
As an example, suppose it is desired to
determine the trajectory data for a type 8
projectile where the velocity is 500 fps, given
an initial (muzzle) velocity of 1300 fps. The
procedure for evaluating Eqs. 4-59 through
4-63 only involves looking up the values of p,
t, h, and q x for u = S 00 fps and 1300 fps
from the Siacci Table based on the drag
function for a type 8 projectile G t and then
solving Eqs. 4-59 through 4-63 for these
values.
Inclination:
£_ f 2,T dM
a 0 Jj, Af ? C fl (Af) *
dimensionless ( 4 - 66 )
Altitude:
where
A
I(M)dM
mc d m •
dimensionless (4-67)
gla 0 = 32.154/1120.27 = 0.0287 sec *
a 0 * speed of sound in air
= 1120.27 fps
In terms of the modified Siacci functions,
the trajectory equations are as follows (again
neglecting variations in air density and Mach
number):
X = (l/y)(S, - S io ) 3 ft (4-68)
‘f - Ft - n„). mo (4-eo»
when the subscript o associated with the
Siacci functions indicates initial conditions.
* =x tan 6 0
U-AJ
For fin-stabilized projectiles, the Siacci
functions are modifieu based on the drag
coefficient for the 900 mm HEAT, T108
Projectile, which has rigid fins. These
modified Siacci functions as a function of
Mach number M are:
Space:
Time:
c = f* ,T dM
* Jjr AfCjjtAf) *
dimensionless (4-64)
r = f 2 ‘ 7 dM
1 J„ WcjM’
* x ^rY- * (4 - 70,
tan 9 = tail 0 o - ^!fL_£^(/ _ J 0 ) ( 4 . 71 )
and for z = 0
<4 -™
V = aJA cos 9J cos 9 (4-73)
where
dimensionless (4-65) y = p^i t d 2 /W, ft ' 1
4-32
AMCP 700438
As an example, the following problem is
presented:
1 . Problem: To find the velocity, time
of flight, angle of depar¬
ture, and angle of descent
at a range of 1000 yd for
90 nun HEAT, T108E20
Projectile
M S T A L
1.73650 3.09196 1.427502 0.02537 0.01923
2.14234 1.70836 0.709983 0.00670 0.008S1
Difference: 1.38360 Q.7I7S19 0.01867 0.01072
Calculating M 0 and y
2. Given Data:
M 0 -- V m la Q = 2400/1120.27 = 2.14234
(4-74)
Projectile Caliber d
Projectile Weight W
Muzzle Velocity V M
Form Factor i t
= 0.29525 ft
= 14.21b
= 2400 fps
» 1.00 based on C T10 >
y = pgi,d 2 IW - 0.07513 (1.00X0.29525) J /14.2
= 0.0004612 ft'*
Substitution of the Siacci function values
and the value of M 0 and y into Eqs. 4-68
through 4-73 give the following results at a
range of 1000 yd:
Air Density p‘ Q * 0,075) 3 lb-ft' 3
Speed of Sound
in Air * 1120.27 fps
3. Solution:
From the Table of Siacci Functions
based on 90 mm HEAT, T108 (Ref. 10)
Angle of
departure: 0 o = 9.82 mils (Eq. 4-72)
Angle of fall: 0 = 11.32 mils (Eq.4-71)
Tangential
velocity: V = 1945.37 fps (Eq. 4-73)
Time of flight: tf = 1.389 sec (Eq. 4-69)
4-33
REFERENCES
1. AMCP 706-242, Engineering Design
Handbook, Design for Control of Projec¬
tile Flight Characteristics.
2. AMCP 706-140, Engineering Design
Handbook, Trajectories, Differential Ef¬
fects, and Data for Projectiles.
3. R. R. Rhodes, Simplified Exterior Ballis¬
tic Equations, Memorandum Report
MR-672, Frankford Arsenal, Philadel¬
phia, May 1958.
4. Gi-Table, BRL File N-I-92, Aberdeen
Proving Ground, 1945.
5. G a - Table, BRL File N-I-20, Aberdeen
Proving Ground, 1944. G a kl -Table, BRL
File N-I-93, 1945.
6 . M. E. Harrington Table of G t , BRL File
N-I-68, Aberdeen Proving Ground, 1943,
Table of G,, BRL File N-I-96, 1945.
1. Aircraft Table Based on G i, BRL File
N-I-12'2, Aberdeen Proving Ground,
1955.
8 . Aircraft Table Based on G a a , BRL File
N-l-121, Aberdeen Proving Ground, 1955.
9. Aircraft Table Based on G 81 , BRL File
N1-126, Aberdeen Proving Ground, 1955.
10. C. T. Odom Drag Coefficient and Siacci
Functions for a Rigid Fin Shell Based on
the Shell 90 mm HEAT. T108, BRL
MR-882, Aberdeen Proving Ground,
1956 Revision.
11 . David E. Walters and Edith F. Reilly,
Hitting Probabilities of the Standard
Recoilless Weapons, Memorandum Re¬
port M59-32-1, Frankford Arsenal, Phila¬
delphia, Pa., June 1959.
AMCP 706-238
CHAPTER 5
INTERIOR BALLISTICS
r.
= discharge coefficient of nozzle,
5-0 LIST OF SYMBOLS
W
dimensionless
A
A c
A e
A,
AY
= bore area, in?
= chamber area, in?
» nozzle exit zirczi, in.
s nozzle throat area, in.
= surface area of gun being heated,
in?
= unoccupied chamber volume,
v c - C,lp, in?
C a
C s
<Y
- initial propellant charge, lb
= hypothetical charge of “ideal”
rifle, lb
= specific heat at constant pres¬
sure, (ft-lb)-(lb- R) 1
= total weight of unbumed propel¬
lant ejected, lb
= specific heat at constant volume,
(ft-lb>(lb-°R)' 1
a
a
a
p
«i
a 2
B
b
C
C
C"
= l-X/(2t/), dimensionless
= constant term in burning law
equation, in.-sec 1
= peak acceleration, ft-sec" 2
• AC s Km’V b ), in?-see' 1
\ppl<li m , dimensionless
- effective burning rate constant,
in.-(sec-psi)' 1
- X(7-l)/2, dimensionless
= effective propellant charge e b
weight and C — Cf — 1V S , lb
C C
= burning rate coefficient in linear¬
ized burning law equation,
in.-fsec-psi)-' e 2
x burning rate coefficient in non¬
linear b ming law equation,
in.-sec* 1 tpsi)*"
C 2
D
D 0
= specific heat of weapon,
BtU-(lb wca pon
= propellant charge burned in nfie,
Cj = Q - C f , lb
= bore diameter, in.
= initial grain diameter, in.
= perforation diameter, in.
= 2.718281828 base of natural
logarithms
- ballistic efficiency, dimensionless
= thermodynamic efficiency, dimen¬
sionless
= F
r
, fps
= propellant impetus, (ft-lb)-lb 1
5-1
AMCP 706-238
= gas force, PA , lb
- KC d , sec" 1
F n = multiplying factor for converting
7-perforated we’ o to equivalent
single-perforated webs, dimen¬
sionless
/ = fraction of web unbumt, W/W 0 ,
dimensionless
f(u) = (1 + u) (i + ">/“/( 1 + 2 u ) (1 + 2u)/u
f(K V m ) = , dimensionless
WW’
= acceleration due to gravity,
ft-sec -2
= (1- J a ) X/2, dimensionless
= 1- g 0 r 2 2 , dimensionless
= constants in the form function,
N/C = k a + kj + k 7 f 2 , dimen-
sionless
= thermal conductivity,
Btu-(in? -sec- 0 F/in . J" 1
= travel of projectile at any time,
« travel of projectile when propel¬
lant is all-burnt, in.
= travel of projectile to muzzle, in.
= travel of projectile when peak
chamber pressure occurs, in.
= initial length of propellant grain,
= Heaviside function
= heat transfer coefficient,
Btu<in?-sec-°Fy l
« hot'/(kt w ) »sec ” 1
-- /iw/(/c/), dimensionless
- heat transfer coefficient at inner
wall, Btu*(in?«sec °F)‘ 1
- hA w l(RfC w W w ) t round" 1
= heat transfer coefficient at outer
wall, Btu-(in?-sec-°F)" 11
~ fractional momentum unbalance
factor, dimensionless
= 0 for F 0 = 0
= nozzle coefficient, sec"
(r+ iy(T-i)
= yg. ( 1 \
F \7 + V
= weight of projectile, lb
= effective projectile weight, lb
(Af = mg)
= 6 0 !9 X , dimensionless
= mass of projectile, slug
= effective mass of projectile
(1 - \)C: l ,
= 1.04 m + -— 1 .slug
= mass of gases flowing through
nozzle, slug
= weight of propellant burnt, lb
•- weight of propellant burnt at
projectile start, lb
~ weight of gas in rifle, lb
= weight of gas in rifle at all-burnt,
lb
AMCP 706-238
n
n
i
n
n'o
P
P‘
Ft
Pc
Pe
Pm
Po
P P
P,
P x
Q
Qo
Qo
Q
<lo
pressure exponent in burning law
equation, dimensionless
R
round number, dimensionless
R f
number of grains in gun at any
R‘
time
R,
initial number of grains in gun
T
space mean pressure at any time,
f
psi
r
pressure at specific point of con¬
sideration, psi
Y
space mean pressure at time
i
charge is all-burnt, psi
chamber pressure, psi
s
exit pressure at nozzle, psi
spai mean pressure when pro¬
s g
jectile is at muzzle, psi
s starting pressure, psi
; maximum pressure, psi
T
: pressure at nozzle throat, psi
T
: pressure at projectile base, psi
(AV /CA
\C/ \m/ .dimensionless
T'
T a
z heat influx input per round,
T 0
Btu-(in? -round)' 1
Q 0 w/(k T 6 0 t 0 ), round' 1
To
= m K^(p~4 in ’ SKfr '
Ti
= heat transferred to weapon per
round, Btu-fround)' 1
T w
= universal gas constant,
(ft-lbXlb^Rr 1
= rate of fire, rounds min" 1
s W 0 /% 0 j dimensionless
= gun tube radius, in.
= instantaneous burning rate,
in.-sec" 1
= radial distance into wall for heat
transfer equations, in.
= C 2 KpA), in.
= effective burning rate, in.-sec' 1
= total area of the propellant
charge burning surface, in?
= surface area of single-perforated
grain, in?
= fraction propellant loss, CJC l ,
dimensionless
= space mean temperature at any
time, °R
= average gas temperature during
ballistic cycle, 0 F
= T/T 0 y dimensionless
= air ambient temperature, °F
= isochoric flame temperature of
propellant, °R
= maximum temperature at inner
wall, °F
= temperature of gun tube after
firing first round, °F
= wall temperature of gun, °F
5-3
\ ...~~
.... v ^ $&?>*** ■ -W* 8
1
1
AMCP 706-238
•’J
■ 0 *
!
3
i
1
= time, sec
w
= wall thickness, in. ^ |
A
1 0
= w 2 sec
= wall thickness corresponding to J
flf
pressure at all-burnt, in. |
u
= 1 — by dimensionless
|
= wall thickness cofresponding to j
&
U
-f
peak pressure, in. i
]
dimensionless
X
= effective distance of projectile to !
£./
7
F„
W/L
5-4
= velocity of projectile at any time,
fps
= velocity of projectile at all-burnt,
fps
= muzzle velocity of projectile, fps
= velocity of gases flowing through
nozzle, fps
= velocity of projectile at time of
peak chamber pressure, fps
= chamber volume of rifle, in?
= volume of all single-perforated
propellant grains, in?
= A(L + x 0 ) - ( C) - N)/p = free
volume in gun, in?
= web thickness of a burning grain,
in.
= weight of gun tube, lb
= initial web thickness of pro¬
pellant grains, in.
= weight of unburned propellant
ejected, lb
= weapon weight, lb
= web thickness of 7 -perforated
grain, in.
» effective propellant charge re¬
gressiveness, dimensionless
x b
x n
p
Y
Y'
oc
a'
0
breech such that Ax is the volume
behind the projectile, L + x 0 , in.
= x 0 +L b , in.
= effective length of chamber such
that Ax a = v c , in.
= effective length of rifle such that
Ax m is the total volume of rifle,
(i.e., Ax m - p c + AL m ), in.
= x D +£ p , in.
= unoccupied chamber length =
x a - Cjl(pA), in.
= x/xj, dimensionless
= Cjl(pv e ), dimensionless
= diffusion constant, in?-sec" 1
= ratio of heat loss to kinetic ener¬
gy of the projectile, dimension¬
less
= ratio of specific heats,7 = c p /c v ,
dimensionless
= pseudo ratio of specific heats, 7
= (1 + 0)(y — 1) + 1
= yC d K
, sec
.-1
= [(7 - 1 ) X + 1 ] , dimensionless
= initial solid propellant loading
density, 27.7 C t /v c , g-cm' 3
1
1
•mm 11 ' Mumvmm
TSSSSSBg^Sm
AM CP 706-238
S
£
r
f 2
T?
0
0 (n)
iT
0'
0,
Oo
e t
x
p
= (c - ffl)/(l - a), dimensionless £
.. Total Gun Volume v
= ex Pansion ratio, ‘^amberVolume’ *
dimensionless
v
_ (1+fl) (7~ DM \
2*F
X, sec-ft 1
_ \ 2 nxVj (x^/x j ^ .dimensionless
v c Pb 0l (2-7)
= covolume of propellant gases,
in?-lb -1
= 1 - \(T 0 /T ) 1 n , dimensionless
= wall temperature above ambient,
6 = T- T a or
0 (m) = 0,11 -exp(-nft„)l,°F
P
p'
a
T
T
t r
= i _ XT 0 /T) in up to burnt;
Vj/Cj after all-burnt, dimension- 4 >
less
<P
= N'/N, dimensionless
4 >'
= temperature decay at wall after
initial temperature rise, °F <t > 2
0 d = 6 1 exp [ — hA w t/
= r/w, dimensionless
= Ax/(v c - C/p), dimensionless
= P(v c - Cj/p)l( 12 CjF), dimen¬
sionless
= density of propellant, lb-inT 3
= density of gun material, lb-ini' 3
= allowable tensile strength of the
material, psi
= t/t Q , dimensionless
dimensionless
= dimensionless time between
rounds = 1 l(t 0 Rf)
= 0/0 o , dimensionless
= 1 — \yUI 2 - f V, dimensionless
= N/C \, dimensionless
= 2 (g, - N b /C 2 )/(N b /C 2 ), dimen¬
sionless
= equilibrium temperature, °F
^ = 1 l<j>2 + 1 > dimensionless
= gas temperature above ambient,
deg F
= maximum temperature rise at in¬
ner wall, °F
= 0(1) - 0(0) = 0,[1 - exp
(- h n )], °F
= kA,W Q l(C 2 B), dimensionless
= piezometric efficiency, =
12 (‘/2)m F 3 l(P p AL m ), dimension¬
less
\j/' b = value of tf/' for V - F ft , (fps) -1
\p' m = value of \p' for V - V m , (fpsf 1
\jj' a = value of \j/' for V- 0, (fpsF 1
= Vl2 + l(4i' 0 l2) 2 +Wb - KWb\
u) = fraction of propellant web burnt,
u) ~ 1 — dimensionless
52 = (N - N')/‘ N, dimensionless
5-5
AMCf» 706-238
SECTION I
INTRODUCTION
5-1 SCOPE
The theory of interior ballistics provides
the bases for the calculation of pressure
within the gun and projectile velocity as
functions of projectile displacement. Those
factors which affect projectile motion in the
gun are within the scope of the subject of
interior ballistics. Many contributory factors
such as the theory of propellant burning are
the same for recoille&s weapons and conven¬
tional guns, and hence are covered in other
references given at the end of this chapter.
The material in this chapter provides an
understanding of the interior ballistic pro¬
cesses and the relationships among interior
ballistic parameters and weapon system char¬
acteristics. Approximate solutions and graphi¬
cal methods are presented which allow the
designer to estimate quickly the effects of
these relationships, however, a digital com¬
puter program as outlined in Section VII is
recommended for more accurate calculations.
5-2 QUALITATIVE DESCRIPTION OF THE
INTERIOR BALLISTIC PROBLEM
The interior ballistics of a recoilless rifle is
a complex subject, and it is helpful to obtain
a qualitative understanding of all factors that
influence the motion of the projectile before
undertaking the detailed mathematical analy¬
sis. All appropriate thermodynamic constants
are space averaged.
The projectile is accelerated by the propel¬
lant gas pressure acting on its base. The
instantaneous pressure in the gun is deter¬
mined by the amount of propellant that has
burned, the amount of propellant gas that has
been discharged through the nozzle, the avail¬
able volume behind the projectile into which
propellant gas expands, and the temperature
of the propellant gas. The pressure is then
determined by use of an appropriate equation
of state.
The rate at which propellant bums is a
function of the gas pressure, the amount of
burning propellant surface, and the density of
the solid propellant. The surface area is
determined by the geometry of the propellant
grain and the number of grains or total weight
of propellant that is burning. The propellant
grain is typically cylindrical in shape and its
burning surface is controlled by the number,
distribution, and diameter of holes through
the length of the grain. The integrated propel¬
lant burning rate determines the amount of
propellant burnt at any time.
The rate at which gas is discharged through
the nozzle is determined primarily by the
pressure in the rifle, the weapon, the
geometry of the nozzle, and to a lesser extent
the temperature of the propellant gas. The
configuration of the nozzle, of course, deter¬
mines the recoil thrust and must be designed
to eliminate the net system recoil.
The gas temperature in the gun is a
function of the particular propellant used, the
effects of gas expansion, and heat conduction
to the gun wall.
In a later section, these processes will be
described quantitatively by means of a set of
simultaneous differential equations yielding
the gas pressure and projectile velocity as
functions of projectile displacement. The gas
pressure influences the design wall thickness
of the gun tube; the desired muzzle velocity
determines, in part, the length of gun tube.
The wall thickness and tube length largely
determine the system weight of a given caliber
rifle.
5-7
Preceding page blank
AMCP 706-238
The typical interioi ballistic problem is the
determination of the complete set of ballistic
parameters which will lead to the optimum
gun design (usually the lightest gun) that will
provide the projectile with the specified en¬
ergy.
This qualitative discussion is illustrated in
Fig. 5-1 that presents schematically an ideal¬
ized recoilless rifle and an equivalent recoilless
rifle showing the interior ballistic parameters,
each consisting of a tube of cross-sectional
area A with an orifice at one end of throat
area A t . In the equivalent rifle, the initial
position of the projectile is at a distance x 0
from the throat giving a chamber volume
v c - Ax 0 . The instantaneous gun volume be¬
hind the projectile is Ax.
5-3 USE OF EXISTING REFERENCES ON
INTERIOR BALLISTIC THEORY
There is no convenient, closed-form solu¬
tion to the set of the differential equations of
interior ballistics. Approximate solutions have
been obtained by making appropriate simpli¬
fying assumptions. There are a number of
different methods for solving the ballistic
equations, such as Comer’s or Hirschfelder’s,
which are indicated in the references. These
procedures are available, effective, and inter¬
esting. However, graphical and analytical
methods of solution will be presented in
sufficient detail to provide choices based on
required accuracy. These choices range from
simple graphs of dimensionless parameters for
quick approximate solutions to more precise
digital computer solutions of the basic differ¬
ential equations.
5-4 DESIGN DATA FOR SEVERAL RE¬
COILLESS RIFLES AND AMMUNI¬
TION
A simple and useful method for predicting
the performance of a conceptual recoilless
rifle is the comparison with the performance
of a similar existing rifle of known character¬
istics. The conceptual design parameters are
estimated through the application of
TABLE 5-1
BALLISTIC PARAMETERS FOR SEVERAL GUNS AND ROUNDS
Gun
57 mm M18
75 mm M20
90 mm M67
105 mm M27
106 mm M40
Round
M306A1
M309A1
M371
M323
M344
L„, in*
m
47.5
65.1
27.5
106
109.8
Pp. psi
7,500
10,000
3,700
9,260
10,280
m, slug
0.0854
0.4361
0.42
1.006
0.544
V m > f P s
1200
990
450
1120
1650
C-, lb
1.00
3.309
1.31
7.87
7.46
Type Propellant*
M10
M10
M5
M10
M10
A, in?
3.96
7.00
10.1
13.72
13.72
A t , in?
2.95
4.67
6.82
9.30
10.00
V in.
32.8
40.9
14.8
60.9
32.4
Web, in.
0.0179
-
—
0.0336
0.035
Grain
Single Perf
-
—
Multi-Perf
Multi-Perf
, c ,in?
120
286
150
840
444
V*r
2.39
2.07
3.5
1.88
1.79
A/A
1.34
1.49
1.48
1.47
1.38
•See Table 11-3, Chapter 11, "Ammunition", for Propellant Parameters
5-9
AMCC 706-238
similitude relationships. These methods are
discussed in the next section.
Table 5-1 presents design parameters and
performance details for a series of existing
recoilless rifles to aid the designer of new
systems in scaling the ballistic parameters.
These data are useful also for checking the
accuracy of an analytical solution or of a
digital computer program of the ballistic
equations.
AMCP 706-231
SECTION II
EMPIRICAL AND GRAPHICAL METHODS
FOR QUICK APPROXIMATIONS
5-6 SOLUTIONS BASED ON EFFICIENCY
CONSIDERATIONS
55.1 INTRODUCTION
The methods of this section are based on
the observation that if an existing gun yields a
certain projectile muzzle energy pei unit
charge, then other guns of the same efficiency
can be designed to meet different require¬
ments of performance and size. These meth¬
ods do not provide detailed gun design data
but are useful for rapid estimates of the gross
dimensions (weight, length, volume) and pro¬
pellant requirements.
5-5.2 THERMODYNAMIC EFFICIENCY
For conventional guns, thermodynamic ef¬
ficiency e c is defined as the ratio of the
projectile kinetic energy to the available
energy of the propellant. In the recoilless rifle
system, however, some of the useful available
propellant energy goes into balancing the
rifle-i.e., preventing recoil-so that in com¬
paring fairly identical recoilless rifles by using
the conventional efficiency definition one
rifle will appear more efficient and require a
smaller charge when it recoils rearward, and
less efficient when it recoils forward. There¬
fore, in order to compare effectively the
ballistic performance or potentiality of re¬
coilless systems, the efficiency of a recoilless
rifle is now defined as the ratio of useful work
obtained from the system to the available
propellant energy (Ref. 1). The useful work
from the system is the kinetic energy of both
the projectile and nozzle gases. Considering
the case of an “ideal” rifle, i.e., a rifle in
which there are no energy losses, the ballistic
efficiency e b of the recoilless rifle can be
written as
e b = C 0 /C it dimensionless (5-1)
where
C 0 = hypothetical charge that contributes
to the energy of the projectile and
the gases balancing the recoil forces,
lb
C t = initial propellant charge, lb
Considering the “ideal” rifle system, the
conservation of energy and momentum can be
written, respectively, as
m„V 2 „/ 2 + mV 2 m /2 = C^/iy - 1) (5-2)
m„V n + ImV m = mV m (5-3)
where
7 -• specific heat ratio, Cp/c^,, dimension¬
less
F = propellant impetus, (ft-lbHb -1
/ = fractional momentum unbalance
factor, dimensionless
m ~ projectile mass, slug
rn„ = mass of gases flowing through noz¬
zle, slug
V„ -velocity of gases flowing through
nozzle, fps
V m = muzzle velocity of projectile, fps
The term C„F/(y-l) is the total available
5-11
AMCP 706-238
propellant energy of the ideal propellant
charges and Im V m is the momentum of the
rifle and accessories: / is defined as the
fractional momentum unbalance factor which
is positive for rearward recoil and negative for
forward recoil.
Solving Eq. S-3 for V n and substituting into
Eq. 5-2, and then solving the resulting qua¬
dratic equation for C 0 , the ballistic efficiency
e b of a recoilless rifle becomes as shown in
Ref. 1.
(5-4)
It should now be noted that for a closed
breech weapon / = 1 and e b reduces to the
conventional thermodynamic efficiency defin¬
ition e c ,
„ _ mVjiy - 1 )
6 5(5-4a)
5-5.3 PIEZOMETRIC EFFICIENCY
Piezomeiric efficiency is the ratio of the
equivalent constant pressure (average pres¬
sure) of the ballistic cycle to the actual peak
pressure of the system. Hence, piezometric
efficiency p is
= pT" \ L * P{JL)dL (5-5)
■fV-'m Jo
From the equation of motion, force equals
mass times acceleration
pu - u - -(f)
and from
it follows that
P(L)A = Umv(^j (5-5a)
Evaluating the integral of Eq. 5-5
r—■ “iT w - i2 (x)(4)
(5-5b)
Substituting Eq. 5-5b into Eq, 5-5, we obtain
fx =12
(5-6)
where
P = space mean pressure at any time, psi
P p - maximum pressure, psi
L m = gun barrel length, in.
A = bore area, in?
Peak pressure is a significant design param¬
eter affecting gun weight, blast, and flash.
Thus, a characteristic piezometric efficiency
provides a quick estimate of tube length given
a required projectile weight and velocity, and
a specified allowable maximum pressure. The
larger the piezometric efficiency, the shorter
the travel sequired for a given muzzle energy.
Hence, an important consideration in the
determination of the propellant charge and
gun design is the maximizing of the area
defined by the pressure-travel relationship,
while attempting to minimize both tube
length and peak pressure.
5-5.4 EFFICIENCY TABLES AND GRAPHS
Table 5-2 lists values of thermodynamic
and piezometric efficiencies for some existing
recoilless rifles. These efficiencies correspond
to the systems identified in Table 5-1. This
5-12
*'*??• v ,|. a p g , , .- w *
QV’'>*V' **».-»iqiv.f
AL9CP 706-238
i
!
!
>
t
r
t
TABLE 5*2
P1EZQMETFJC, BALLISTIC. AND THERMODYNAMIC EFFICIENCIES
OF SOME EXISTING RECOILLESS R2FLES
Conventions*
PwMKMtric Thtrmojivnimic Bdiiitic
Weapon
Round
Efficiency n
Eftic&iicy 0 .
Efficiency
57 mm MIG
M306A1
0.52
0.052
0.44
75 mm M20
M309A1
-
—
0.50
105 mm M27
M323
0.55
0058
0.54
106 mm M40
M344
0.56
0.071
0.47
table indicates that a typical, well designed
recoilless system should have a ballistic effi¬
ciency e b and piezometric efficiency /i of
about 0.50, and a conventional thermo¬
dynamic efficiency e c of 0.06.
&££ NUMERICAL EXAMPLE
Assume that a 105 mm, 8-lb projectile is to
be launched at V m = 2000 fps with F = 3.3 x
10* ft-lb/lb and that peak pressure P p is not
to exceed 10,000 psi. Applying a convention¬
al thermodynamic efficiency c c of 0.06 and y
* 1.24, one calculates from Eq. 5-4a
hence
It is often convenient to use dimensionless
coefficients instead of the ballistic parameters
themselves. Fo; example, by review of empiri¬
cal data, the dimensionless propellant weight
coefficient (propellant weight per unit pro¬
jectile weight) is found to be closely approxi¬
mated by a single valued function of projec¬
tile velocity. This relation appears in Fig. 5-2
with the two curves representing two values
of ballistic efficiency e b (0.4 and 0.5).
C
_ (Y - 1 )&)mVl
(0.06) F
<0.24)tl)(a»(2000)»
(0. 06X3.3 x 10 6 )
Fig. 5-3 shows the relation between
chamber volume and both projectile travel
and propellant weight through the use of the
dimensionless coefficients-expansion ratio c
and loading density A 0 .
* 6 lb of propellant
A piezometric efficiency p of 0.5 yields, using
Eq. 5-6,
= 12 x 10 2 in?
= 13.4 in? (.or 105 mm projectile)
Fig. 5-4 is the projectile travel L m required
to obtain a specified muzzle velocity for given
values of peak acceleration based on Eq. 5-7.
_ r M
= 2^r* in *
where
(5-7)
V m = muzzle velocity, fps
a p = peak acceleration of projectile,
ft-scc* 7
ix = piezometric efficiency, dimensionless
Piezometric efficiency p is assumed to be 0.60.
AMCP 700-233
54 TABULATED DESIGN DATA
54.1 METHOD
In Ref. 2, a series of interior ballistic
calculations have been made and the results
tabulated. These data arc shown in Table
5-3(A). If the projectile weight and bore area
are specified, this table can be used to
estimate various combinations of chamber
volume, barrel length, and peak pressure to
produce a specified muzzle velocity. Note
that there are three sets of calculations
corresponding to propellant loss of 0. 10, and
20 percent. The choice of most appropriate
data will be at the discretion of the designer
based on the nozzle design and the point
where propellant burning ends. Tire method
for using the table follows:
5-14
AMCP 706 *230
.H'
X /
• Ct ''
"■■if
, :
I-
#
Bore Area A times Travel L, in"
Figure 5-3. (A) Lower abscissa: Chamber Volume as a Function of Propellant
Weight for Loading Densities 0.4 , 0.5 , 0.6 g-cm~ 3
(B) Upper abscissa: Chamber Volume as a Function of Barret Volume (Bore Area Times
Travel) for Expansion Ratios 2, 3, 4, and 5.
Given V m .m\ A . and P p :
(1) Compute ratio m'/A where the m is the
effective projectile mass (see par. 5-10).
(2) Choose a set of values for the flow
factor X (see par. 5-7.2) in the range 0.45 <X
<0.65.
(3) From Table 5-3(A), for each X in the
set of Step (2), read L m (mlAF l and i > c /m'
corresponding to the given maximum pressure
and muzzle velocity. This should be done for
each propellant loss; 0 percent, 10 percent,
and 20 percent.
(4) Compute the total travel L m for each X
5-15
AMCP 706*238
Figure 5-4. Muzzle Velocity as a Function of Projectile Travel in the Barrel for
teak Projectile Acceleration 2, 500, 5,000, 7,500, and 10,000 g's.
and propellant loss by
(h
\A
(5) Compute the chamber volume v c by:
( !2Z] A
m
(6) Tabulate results L m and r c versus X
(7) Determine the value of C t for the vari¬
ous values of X from:
Ci = •
100C,
* 100 — % propellant lose
where
= fe) ™'8
5-16
j/a-
I-Jr'
AMCP70G-23B
VK
m
m
W,
r W
#■
•r.
#'
: .,r‘
v \
a.
w m t
X jpt
0.45 1500
2000
TABLE 5-3 (A)
G5.NP.RAL BALLISTIC DESIGN DATA BASED ON SIMPLIFIED THEORY
P. v e /fn\ L m (a). v m,
in?/slug Iw? /slufl ^ JP}
pst
Powder Loss - 0 percent
3000
0.50 1500
2000
3000
0.55 1500
2000
0.60 1500
2000
0.65 1500
6000
10000
15000
20000
30000
6000
10000
15000
20000
30000
15000
20000
6000
10000
15000
20000
30000
6000
10000
1500o
20000
15000
6000
10000
15000
20000
6000
10000
15000
6000
10000
15000
6000
10000
6000
10000
1575
1003
717
574
431
1708
1113
815
668
521
1035
874
1109
729
528
431
334
1209
812
613
513
832
805
538
404
337
910
630
490
581
403
314
701
503
421
307
3180
1997
1238
926
600
5267
3132
2065
1537
1010
6605
4966
2991
1682
1177
871
571
6201
3148
2087
1572
7253
2895
1722
1146
841
6140
3670
2430
3429
2067
1391
8922
5442
4938
3018
0.46 1500
2000
3000
0.50 1500
2000
3000
0.55 1500
5!*
In?/slug
Powder Loss 1,1 1
6000
1758
10000
1114
15000
799
20000
638
30000
478
6000
2022
10000
1309
15000
950
20000
713
30000
595
15000
1261
20000
1053
6000
1311
in?/ring
) percent
10000
15000
20000
30000
6000
10000
15000
20000
15000
20000
6000
10000
15000
848
617
501
375
1548
1020
757
626
1029
873
990
653
495
3392
1956
1291
932
622
5758
3581
2270
1722
1136
4867
3299
1962
1298
965
595
5682
3270
2183
1626
7227
5447
3170
1917
1325
20000
400
950
2000
6000
1172
6428
10000
795
3845
15000
605
2555
20000
511
1929
1500
6000
767
3573
10000
518
2102
15000
394
1406
2000
6000
919
8500
10000
639
5145
15000
504
3525
1500
6000
577
4425
10000
404
2685
2000
10000
534
7402
AMCP 706-238
$1
<,9ff
f . ■
: ■ 0
...
.***$■:■
K V
- -
TABLE 6—3(A)
GENERAL BALLISTIC DESIGN DATA BASED ON SIMPLIFIED THEORY (CONCLUDED)
.. 1
LS&L. £
V
v c /m',
.. «T.
Km '
V
v c /m.
■~m ■ )
^ fps
p«
in?/slug
in?/slug
1 X
fP*
&
In? /»lug
in?/slug
Powder Loss
■ 20 Percent
■**'i
0.45 1500
6000
1927
4013
0.55
1500
6000
1220
3860
$
10000
1231
2401
10000
800
2280
■ A,
15000
682
1558
15000
590
1530
,W*\
20000
708
1132
20000
485
1141
■j®
30000
533
760
30000
380
759
2000
6000
2348
6812
2000
6000
1492
7278
? ^ ,
10000
1516
4014
10000
997
4363
' P. t
15000
1101
2669
15000
750
28 1 8
-VX- ;
20000
893
1971
20000
626
2199
30000
686
1289
'abrwV' V'.
3000
15000
1516
8134
3000
20000
917
7082
'f
20000
1257
6063
30000
996
4054
0.50 1500
6000
1473
4017
0.60
1500
6000
993
3937
10000
954
2386
10000
658
2392
»|
15000
695
1575
15000
492
1592
fUL.
20000
566
1164
20000
410
1193
. 4f-
30000
436
756
&
2000
6000
1805
6775
2000
6000
1224
7762
, v p^af-
10000
1188
4102
10000
832
4852
15000
880
2710
15000
637
3262
,r
20000
725
2025
20000
539
2462
30000
571
1339
0.65
1500
6000
788
4497
3000
15000
1234
7537
10000
538
2772
jjf*
20000
1041
5687
15000
413
1868
2000
101)00
707
6440
15000
553
4400
,r ' k
The ratio
C 2 /M’ of
charge burned C 2 to
m' =
Q.34 slug
effective projectile weight M is obtained from
3 i ,e :
Table 5-3(B).
m'/A =
0.05 slug/in?
5S2 EXAMPLE
Consider a 75 mm gun with a round which
has the following parameters:
P p = 10,000 psi
A = 0.85 in?
V m = 2000 fps
From Table 5-3(A) for X
propellant loss
vjiri = 812 in? /slug
and
Ljm'/AV* =3148 in? /slug
= 0.5 and zero
5-18
AMCP 706-238
TABLE &*3(B)
and, since zero propellant loss was assumed,
TABLE OF PARAMETERS BASED ON
SIMPLIFIED THEORY
100C 2 100 y
Ci ~ 100-0 ~ 100 4# 99)“ 4.89 lb
\
v m = 1500 fp*
Ci/M 1
0.45
0.316
0.50
0.314
0.55
0.310
0.60
0.305
0.65
0.301
0.45
u = 2000 fps
m
0.460
0.50
0.456
0.55
0.450
0.60
0.440
0.65
V m = 3000 fps
0.436
0.45
0.788
0.50
0.777
0.55
0.757
0.60
0.745
0.65
0.723
therefore
v c = (812)(0.34) = 276 in?
and
L m = (3148) (0. 05) = 157 in.
A complete table of combinations of chamber
volume and barrel length is then obtained by
repeating these calculations for the other
values of X, and for 10 and 20 percent
propellant loss.
5-7 GRAPHICAL SOLUTIONS
5-7.1 INTRODUCTION
In this paragraph a step-by-step procedure
is described for determining recoilless rifle
design parameters graphically. The bases for
these graphs are described in Ref. 3.
This graphical method does not permit a
calculation of pressure and velocity as a
function of projectile travel but it does
provide recoilless rifle and propellant param¬
eters that will yield a specified muzzle
energy ana peak pressure. In general, these
graphs are based on the simplified Hirsch-
felder Theory as found in Ref. 4. It is
assumed that V mt m, A, and the type of
propellant are specified. Then A r A e > and
A c are chosen and determined as dimension¬
less quantities such that the rifle will be
recoilless (A/A t usually taken as 1.45, and
A c jA t close to unity). For these conditions
A e is nearly double A r
The propellant charge corresponding to these
values is determined in the manner that
follows.
From Table 5-3(B), C 2 /W' = 0.456
Therefore, since M '= m g
m'g = (0.456)(0.34)02.2)
^ 4.99 lb
Figs. 5-5 through 5-13 contain several
parameters (represented by the following
symbols: X, and \fc) that are dis¬
cussed in Section V of this chapter. Most of
these parameters are of no special interest to
the weapon designer, however, the factor X is
of special interest. It is defined as X =
kA tt W 0 l(C 2 B ) which shows that a specific
value of X determines the propellant charge
C 2 . The use of Figs. 5-5 through 5-13 enables
the weapon designer to estimate a value of X
based on previous experience and then, per-
5-19
AMCf 70*238
!
lOxlO -4
9X10 -4
-4
S, 5x10
Figure 5-7. i/z^/tA/A ,) as a Function of Factor X
forming a minimum number of limiting cal¬
culations, obtain the optimum value of X
which will lead to a practical loading density.
The curves in the figures that follow indicate
the exact value of charge and peak chamber
pressure which correspond to the desired
loading density. With this information, the
chamber volume and propellant web can be
determined, respectively, from the definition
of loading density A 0 and X where
A) = 27.7 C { /v 0 , g-cnf 9 (5-8)
where
C ( - initial propellant charge, lb
5-22
AMCP 706-236
Figure 5-10. Bore Area Times Projectile Travel AL as a Function
of AY and ^V b
approximately 95 percent of the muzzle
velocity V m . However, in order to simplify
the curves, it is assumed that V b - V m with
only a small error being introduced.
5J.2 PROCEDURE FOR USING GRAPHS
Given V m . m. and/4//i,:
(1) Determine C t from Fig. 5-5.
(2) Determine \p b , ^ 0 \ and ^ from Figs.
5-6, 5-7 and 5-8, respectively.
(3) Determine AY from Fig. 5-9 or 5-10.
The scales of both coordinates on these
figures may be simultaneously multiplied by
the same constant factor.
Effective Mass to Peak Pressure Ratio m/P , slug-(psi)
AMCf 70*2»
(fps )' 1
10x10
I i 1 I I < *
o o o o o o 2
*-4 H ir-4 ^
X X X X X X X
© 00 t-* © iO 2
H ri **»
l■■lHlrfinunRMM^M
0 40 80 120 160 200 240 280 320
Unoccupied Chamber Volume AY, in?
8. S x 10
8.0 x 10
7.5x 10
7.0 X 10
6.5 X 10
G. 0 x 10
5.5 x 10
,0x 10
,5x 10
,0 x 10
,Sx 10
, 0 x 10"
, 5 x 10"
.Ox 10"
. 0 x 10"
0 600
Figure 5-12. Effective Mass to Peak Pressure Ratio m /P p
as a Function of AY for ij/' p x 10 4 from
1 to 20
5-27
Effective Projectile Mass *o Projectile Weight Ratio m'/M, slug-lb
Figure 5 - 13. Charge to Projectile Weight Ratio C,/M as a Function of
Effective Projectile Mass to Projectile Weight Ratio m 7M for Values of X from 0.3 to 0.6
w»s*»s.?t»>*rst
AMCP 706-238
(4) Detennine m'/P p from Fig. 5-11 or
Fig. 5-12. The scales in these figures may also
be simultaneously multiplied by a constant
factor as in Step (3).
(5) Determine m' from Fig. 5-13.
(6) Compute P p * m'/(m'/P p ).
(7) Compute chamber volume v c
v c * AY + C t lp (where p is the density
of solid propellant in
lb-in." 3 )
(8) Calculate the loading density A 0
- 2-7 q/v c
(9) A set of recoiiless rifle and propellant
parameters has been determined correspond¬
ing to a particular value of X. The process is
repeated with a change of X to yield another
set of parameters consistent with the required
muzzle velocity and tube length. By tabulat¬
ing and plotting the results of these calcula¬
tions a suitable choice of X will result in an
optimization of peak pressure, chamber
volume, and propellant charge under the
constraints of the weapon system.
5-7.3 NUMERICAL EXAMPLE
Given the following parameters for a 105
mm M27 Recoilless Rifle using an M323
Projectile and M10 Propellant:
A = 13.72 in?
A, = 9.31 in?
A/A t = 1.473
At = 32.41b
V m = 1120 fps
L m =106 in.
F - 3.31 X 10 s (ft-lb)-lb -1
y = 1.24
1 } = 17.09 in?-lb 1
K = 6.46 X 1(T 3 sec' 1
The results of the calculations per¬
formed based on these parameters are
plotted in Figure 5-14 which shows that for a
loading density (0.6 g-cm' 3 ), the factor X
would be 0.585; the charge C t , 9 lb; and the
peak pressure P p , 7500 psi.
5-8 SIMILITUDE RELATIONS
5-8.1 INTRODUCTION
The basic interior ballistic equations as
derived in Section III can be written in
dimensionless form by use of the following
dimensionless variables:
<t>' = N/C t
O =(N-N')/N
O' =n'/n
T > - T / Tg
v = P(v c - CVp)/(12C,F)
I' =Ax/(v c -C l /p)
Substitution of these variables into the basic
equations of par. 5-16 and assuming that drip
= 1 in the equation of state, yields the
following dimensionless governing equations:
(1) Equation of Motion:
5-29
Loading Density a g-cm J initial Propellant Charge C. lb Peak Chamber Pressure
AMCP 706-23B
where
(5-10)
(3) Gas Discharge Equation:
Q = \(T i r in (5-11)
where
0 ^kAiWjiCiB)
(4) Continuity Equation:
(1 - Q) = V
(5) Equation of State:
= *(.i +*')
(6) Energy Equation:
(y - 1X1 + p) (dt'V
2 Q \dfj
(5-12)
(5-13)
(5-14)
known, the performance of a similar gun can
be predicted through the use of the similarity
relations developed by maintaining the same
value for Q and X. In practice, Q is held
constant by preserving the charge to bore area
ratio C ( /A, the relative quickness B/W a ,
impetus F, and projectile sectional density
M/A. If Q is constant and the ratio of bore
area to threat area is also unchanged, then the
gas discharge equation is also unchanged. The
specific heat ratio y is not a widely varying
coefficient and can be assumed constant
within the accuracy of this interior ballistics
model. The ratio fi of heat loss to projectile
kinetic energy also must be constant for
similitude conditions, which is expected, since
similar guns are expected to have similar
fractions of heat loss to projectile kinetic
energy.
Identifying the known system parameters
with a subscript 1, the parameters for the
model system should be as follows in order to
obtai i the same pressure-travel and velocity-
travel solutions:
m/A = m\/A\
C t /A = C^/Ax
A/A t —A^/Af j
Xq - #01
B/W = Bx/Wi
r =t\
It is seen from these dimensionless governing
equations that if the numerical value of the
coefficients Q, X, y, and p remain constant the
solution to the governing equations is the
same; i.e., systems for which these coeffi¬
cients are the same will have identical
theoretical ballistic performance.
5-8.2 CHARACTERISTIC SIMILITUDE RE¬
LATIONS
If the performance of a particular gun is
F =F t
5-9 EFFECT OF BALLISTIC VARIATIONS
5-9-1. INTRODUCTION
In many cases, it is desirable to determine
the effect of the variation of such ballistic
parameters as flow factor, impetus, and burn¬
ing rate on the ballistic performance of a
recoilless rifle-especially as affecting peak
5-31
AMCf> 706-238
pressures and muzzle velocity. For example,
an increase in throat area due to nozzle
erosion will cause a significant increase in
forward recoil and may affect the muzzle
velocity to such an extent that the rifle
becomes useless for operation after a certain
amount of usage. Therefore, it is necessary to
be able to estimate the life of the gun nozzle
based on the variation of muzzle velocity with
throat area. The approximations for peak
pressure and muzzle velocity that follow as
given in Ref. 5 are differentiated with respect
to the designated ballistic parameters and the
effects evaluated in subsequent paragraphs.
llm'eifiu)
r e (l - a)
, psi
(5-15)
V m = ^(l _ 5"“), fps (5-16)
where
/(#) = a +u) it * u)/u /(l +2u) a ** )/u ,
dimensionless
6 = (€ — 0 ) 7(1 — a), dimensionless
y = (1 + PXy - 1) + 1, dimensionless
0 = ratio jf heat loss to burning ener¬
gy of projectile, dimensionless
C d = discharge coefficient of nozzle,
dimensionless
5-9.2 EFFECT OF QUICKNESS FACTOR
B/W 0
The quickness factor is defined as B/W a .
The effect of quickness factor on peak pres¬
sure is obtained by differentiating the log
pressure given in Eq. 5-15 with respect to the
quickness B/W 0 .
dP,
Pi
H(‘•?)-(■
y
2
9
lx +1.5)1
(u + l ) 2 J
„ d(B/W n )
(B/WJ
(5-1'/)
For the usual range of parameters a 1 percent
change of quickness factor produces a change
of peak pressure of 1.5 to 4 percent.
a = C i /(pv c ), dimensionless
€ = expansion ratio, total volume/
chamber volume, dimensionless
The parameters e 2 and u are given by
(rM£
(>-l) .
— +
dimensionless
Fm*,
where
T, flow factor = yC d K
mtr-
sec
The elfect of quickness factor on muzzle
velocity is found by taking the derivative of
the log V m , as given in Eq. 5-16, with respect
to B/W 0 as shown in Ref. 5, Chapter II,
1
d(B/W 9 )
* (B/W 0 )
(5-18)
As a typical example of the effect of quick¬
ness, the 57 mm M18 Recoilless Rifle has a
change in muzzle velocity of 0.8 times the
change in quickness.
5-9.3 EFFECT OF IMPETUS F
The change in peak pressure due to a
5-32
AMCP 706-238
change in impetus is, taking the derivative of
P p with respect to F,
(5-19)
Substitution of typical numerical values in¬
dicates that a 1 percent change in impetus
results in a 1.5 percent change in peak
pressure.
The variation in muzzle velocity is
11
y.~ 1 + |
1 In 6 *
dt'
1
_ 2 u '
f" 2 J
F
(5-20)
For the 57 mm M18 Rifle, it is found that the
change in muzzle velocity is about one-half
the change in impetus.
5-9.4 EFFECT OF PROPELLANT REGRES¬
SIVENESS W/L
Propellant regressiveness is defined as a
fictitious web to length ratio W/L and, in the
case of single perforated grains, is given by:
where
W Q = initial web thickness, in.
= initial propellant grain length, in.
C x - total weight of solid propellant
ejected from rifle, lb
C t = initial propellant charge, lb
A change in net regressiveness W/L causes the
following change in peak pressure as found by
taking the derivative of P p with respect to
W/L ,
(. y - A (u +1.5)1 d(W/L)
v 2 jirnw w/l
(5-22)
For small values of W/L, the term
j_ \c 2 F (b\
*2 [ A [w o J
is dose to unity and u is dose to (y - 1 )/2,
therefore, the effect on peak pressure may be
negligible. A typical value of this coefficient
as given for the 57 mm M18 Rifle is
dPt-l o o ) d(W/Ll
p7" (0 - 8) w/l
The equation relating change of regressiveness
to a change in muzzle velocity is:
For the M18 Rifle this term is negative and
approximately one-half. For longer rifles, i.c..
larger values of 6, the effect of regressivencss
becomes negligible.
5-9.5 EFFECT OF FLOW FACTOR T
'file flow factor T is a significant factor
defined by a combination of ballistic
parameters as follows:
r = y C<t K(^j(Jpj , sec* 1 (5-24)
The effect of flow factor on peak pressure is
AMCP 706-236
determined by taking the derivative of P p
with respect to T and given by,
For a typical case of the Ml8 Rifle, FT/e 2 =
0.75, so that a corresponding change in peak
pressure varies inversely about 1.5 times the
change in flow factor.
The effect on muzzle velocity is deter¬
mined by taking the derivative of V m with
respect to V and is given by.
(5-26)
Since FT/e a is, for example, = 0.75 for the
M18 Rifle, the chawge in muzzle velocity will
be about 0.75 times the inverse change in
flow factor.
\
AM* 706-238
SECTION III
BASIC INTERIOR BALLISTIC EQUATIONS
S-tO EQUATIONS FOR PROJECTILE AC¬
CELERATION
In the absence of friction, the equation of
motion of the projectile
dV =
dt ~ m
(5-27)
where
V = instantaneous projectile velocity, fps
P x = pressure acting on the projectile
base, psi
A = bore area, in?
m = mass of the projectile, slug
/ * time, sec
It is obvious that there must be a greater
pressure in the weapon chamber since the
propellant gas itself must be accelerated by
the difference in pressure between the
chamber and the base of the projectile. There
is an additional, usually smaller, pressure drop
required to overcome the effects of gun-wall
friction against the motion of the gas. This
real drop in pressure is effected by artificially
increasing the mass of the projectile in order
to produce the correct acceleration. The
relation between the space-mean pressure and
the pressure at the base of the projectile is
estimated as
P = (1.04 )P.
Jx , £ zJj £il
L &M J
(5-28)
where
P - space-mean pressure, psi
P x = pressure at projectile base, psi
- propellant charge, lb
6 s (e - a)/(l-a)*3
X s kA t W/(CiB), dimensionless
k ~ KCj, sec 1
The factor fi is plotted as a function of Af/Q
in Fig. 5-15.
Eq. 5-27 can now be written as
By defining an effective mass tn* as
m' = 1.04 [w ■f (5-30)
where the factor 1.04 accounts for friction.
Eq. 5-29 can now be written
dV AP
dt ~ m*
(5-31)
It is seen from Fig. 5-15 that for values of
M/C) larger than one the value of 6 is
approximately 3, and this is the value normal¬
ly used for calculations of m\
5-11 EQUATION OF STATE FOR PROPEL-
LANT GAS
The equation of state for the propellant gas
can be taken with sufficient accuracy as
5-35
AMCP 700-236
P(t>, - ) = 1 2N*RT (5-32)
where
t) * gas covolume, in?-lb" 1
N' - quantity of gas in the weapon, lb
7 * gas temperature, °R
R - gas constant, (ft-lbMlb-^R)" 1
v x * free volume in gun, in?
The gas covolume ij is the space occupied
by the gas when it is compressed to its limit.
The free volume » x in the recoilless rifle is
defined as the total volume behind the pro¬
jectile less the volume occupied by the un«
burnt propellant and can be expressed as
v t = -AOL + xj - (C 2 - N)/p (6-53)
where
x 0 - equivalent chamber length. in.
= VJA
L - projectile displacement, in.
N = quantity of gas produced, lb
p = density of the solid propellant,
lb-in" 3
Introducing the propellant impetus F
F = RT 0 , (ft-lb)-lb' 1 (5-34)
where
T a = isochoric (constant volume) flame
temperature, "R, and is the temper¬
ature which the gases would attain if
all the propellant energy was con¬
verted into the formation and heat¬
ing of gases
R = univeral gas constant (ft-lb)-
(lb-°Rr‘
With these substitutions the equation of state,
Eq. 5-32 becomes
PlA(L +x 0 ) - (C 2 - N)/p - nN'\
= 12A’'FT/T 0 (5-35)
It should be noted that the difference (A/— N 1 )
is the amount of gas discharged through the
nozzle.
5-12 EQUATION FOR RATE OF PROPEL¬
LANT BURNING
The rate at which propellant gas is pro¬
duced is given as
= pSr (5-36)
at
where
p = density of solid propellant charge,
Ib-inT 3
S * instantaneous burning surface area,
in?
r - instantaneous burning rate, in.-scc* 1
The instantaneous rate r is expressed in
general form for propellant burning as
r = a + C'P" (5-37)
where a. C", and n arc constants depending
upon the specific propellant composition. For
the pressures encountered in most recoilless
rifles, the value of C" P n >» a and as a result
the burning rate equation is used with a - 0.
Also, it is found that in recoilless rifles, the
value of n is close to unity and therefore as a
useful approximation the burning rate may be
considered lineal and is expressed as
v = G'P, in.-sec* 1 (5-38)
5-37
AMCP 7G8-23B
with the value of the burning rate Cf in.-
(seo-pa) -1 , chosen to give the best agreement
with Eq. 5-37 for the range of pressure
considered for the specific propellant. The
linearized burning rate Eq. $-38 is shown
plotted in Fig. 5-16 for two different pressure
ranges for M10 Propellant that has the follow¬
ing nonlinear burning rate per Eq. $-37
r * 4.53 x 1 (T*P** t
where
C" * 4.53 x KT 1 in. sec^-psi"** 7
ft = 0.7
In file case of the single perforated grain
where burning occurs normal to all the
surfaces, outer surface, perforation surface,
and both ends, the surface area S for a charge
comprising a single-grain is
Average Pros sure P, psi
Figure 5- 16. Burning Bate as a Function of Average Pressure for M10
Composition Propeilani, Lot FDAP81
5-38
AMCP70t-2M
S = 2 f [<A> - - (d + u>Wj'] \
i-f
+ t[U) 0 - wWj + (d + coW,)] i
3
1
X
•” y
or
S = r(i),+^l+yy |
AT
" jiii. L
- 2 (£)“] 1d -‘
i(5-3 8a)
where
D a = outer grain diameter, in.
d * perforation diameter, in.
W a *(D 0 - d)l 2 * web, the minimum
distance the flame front can bum
through and consume the grain, in.
<t> * fraction of web consumed, dimen¬
sionless
8 0 * initial length of grain, in.
It should be noted that as long as fi D > W Q , the
grain will be consumed when hclf of the
web W a is burnt because the grain is burning
from the perforation outward and simultane¬
ously from the outer surface inward. Ob¬
viously, for the same burning rate, the flame
fronts will meet when half of the web is
consumed.
The corresponding density for the single
perforated grain is
J(D*-d 2 )l 0
(5-3 8b)
where
p * density of the solid propellant,
lb-in.* 3
C 2 * propellant charge burned, lb
Substituting Eqs. 5-38b, 5-38a, and 5-38 in
Eq. 5-36, one obtains
lb-sec" 1
(5-3 8c)
The surface area can be obtained from Eqs.
5-38b and 5-38a as
•*&[(> •*)■
For the condition of constant burning surface
propellant, W 0 /t 0 » 0, which is a good
approximation for a single perforated grain,
Eq. 5-38d can be written as
S = 2C 2 /(pW 0 ), in. 2 (5-39)
and Eq. 5-38c as
It-.*," <5-40)
where
B = 2C % in* -(sec-psiT 1
The quantity5/H^ 0 is known as the propellant
quickness with dimensions (sec-psi) M . Values
of B as a function of peak pressure for M10
Propellant are plotted in Fig. 5-17.
For multiperforated grains the web thick¬
ness is the actual measured minimum thick¬
ness of propellant between perforations. A
factor is shown in Fig. 5-33 which relates
the seven perforated web thickness to an
equivalent single perforated web thickness.
Thus, the equation for gas gener on of a
seven perforated grain is estimated a.
$-39
Effective Burning Rate Constar
Figure 5-17. "Effective" Burning Rate Constant B as a Function of Maximum Pressure P
■'SBugr,
dN
dt
lb-sec" 1
<5-40s)
For further information on specific propel¬
lants and propellant geometries, reference is
made to the propellant section of Chapter 11,
“Ammunition'’.
of throat blockage by ejection of solid propel¬
lant is taken into account, a discharge coef¬
ficient of approximately 0.9 is used in order
to satisfy the mass balance equation.
5-14 EQUATION FOR ACCUMULATION
OF GAS IN GUN
5-13 EQUATION FOR DISCHARGE OF
PROPELLANT GAS THROUGH NOZ¬
ZLE
The amount of gas discharged through the
nozzle is equal to the amount of propellant
burnt minus the gas in the recoilless rifle. Hie
rate of nozzle discharge in pounds per second
is
V (5-41)
where
The rate of propellant gas being generated
minus the rate of propellant gas discharged
through the nozzle is the rate of gas accumu¬
lation in the gun. From Eqs. 5-40 and 5-41
the rate of gas accumulation in the gun
becomes
(5-42)
Define the following dimensionless param¬
eters
X * kA t /(C^B) % dimensionless,
N* * propellant gas accumulation in re¬
coilless rifle, lb
A t * nozzle throat area, in?
and
dimension! eas
k -C d K, sec" 1
K = nozzle coefficient
C d * nozzle discharge coefficient, dimen¬
sionless
P ~ space-mean pressure, psi
y = ratio of specific heats, dimensionless
For an isentropic nozzle, the discharge
coefficient is taken as unity by definition.
However, due to losses in ar actual nozzle,
the discharge coefficient is I than one. In
short nozzles the friction an teat losses are
usually small but, wK*n the additional effect
then the rate of gas accumulation in the gun
becomes
(5-43)
and from Eq. 5-40, Eq. 5-43 can be written
(5-44)
5-15 ENERGY EQUATION
The total energy available by burning N
pounds of propellant is Nc v T a where c % . is the
specific heat at constant volume.
(ft-lbHlb-°R)' 1 .
The total available energy is divided among
the following four applications;
5-41
AMOPNMM
(1) Kinetic energy of the projectile:
hm'Vl
(2) Heat Iocs to the gun:
2 m *
Hie total energy balance equation is then:
NcJ. - tfc v T + yc*k(TjT) in (jrj*n' V
+ (6-47)
where 0 is estimated as shown in Ref. 4 as 5-16 SUMMARY OF EQUATIONS
dimensionless (5-45)
where
T a ■ isochoric flame temperature, °R
D ■ bore diameter, in.
Use the relation
and the average value of 6
(5-48)
(5-49)
A rough approximation of 0 is sufficient.
Values of 0 range from about 0.4 for S7 mm
rifles to about 0.2 for 105 mm rifles. A more
detailed discussion of heat transfer is given in
Section IX.
(3) Nozzle discharge energy:
It is shown in Kef. 4 that the energy
dissipated in the nozzle discharge is
>c.*(r o f) 1/, ^j m'V
(5-46)
where
(T a T) l,i represents the average value of
(T 0 T) xn over the discharge time.
The basic interior ballistic equations then can
be summarized as
(1) Equation of Projectile Motion
From Eq. 5-31
and the definition of velocity
\2V -- — = —
dt dt
(5-50)
(4) Internal energy of gas remaining in
weapon:
N*c y T
(2) Equation of Burning
From Eq. 5-44 and considering the
start condition
5-42
AMCP 706-238
where
S =N„
N’ ■ N 0 < 5 - 6A)
N' = + (1 -
where
iV 0 * weight of propellant burnt at projec¬
tile start, lb
(3) Energy Equation:
The energy remaining in the recoilless
rifle is obtained from Eq. 5-47 as
N‘cJ = NcJ 0 - yc v k(T 0 f) U2 (^jm'V
+ (l+pWy 2 /2 (5-52)
(4) Equation of State
Eq. 5-35 yields
12 F {r )" % ! V X ‘‘ * L) -^r^ ■’’"'l
(5-53)
5-4?
AM CP 706-238
SECTION IV
DISCUSSION OF SOLUTION TO EQUATIONS
In Section III, four equations were ob¬
tained based on the assumptions indicated
and relate the chamber pressure, projectile
motion, gas temperature, and propellant gas
accumulation in the recoilless rifle. This hand¬
book will not present the detailed analytical
solutions to these equations as obtained by
Hirsehfelder or Comer and others who have
simplified the equations by choosing values of
certain propellant, projectile, and weapon
parameters such as \ v f , and W, in order to
obtain analytical solutions of the ballistic
equations. Since these methods involve
lengthy calculations, it is not recommended
that they be used.
By making the simple approximation that
the insi.mtuneous gas temperature T can be
replaced with an average value, the ballistic
equations can be solved by a simple integra
tion that is discussed in detail in Section V.
The solutions obtained by this method give
results within reasonable accuracy so that, for
first approximations, this simplified solution
or the graphical solutions presented in par.
5-7 should be used. For more accurate solu¬
tions to the ballistic equations, a digital
computer program for the numerical integra¬
tion of the basic equations should be
employed.
5-45
Preceding page blank
AMOS’706-238
SECTION V
SIMPLE SOLUTION BASED ON CONSTANT AVERAGE TEMPERATURE
5-17 INTRODUCTION
In order to make the system of ballistic
equations derived in Section III readily inte¬
grate, a gas temperature averaged over the
integration limits is used in place of the
instantaneous gas temperature. With this as¬
sumption, a solution of the ballistic equations
is obtained as shown in the discussion that
follows. The method of solution presented
herein serves as a general procedure for
solving these equations since similar solutions
also can be obtained after making certain
assumptions about other ballistic parameters
and using the outlined procedure.
X
<1
= the average value of T 0 /T
over the ballistic cycla (5-54b)
-kA i W 9 /(CiB) 9 dimensionless
(5-54c)
The resulting solutions are simple and
displayed in such a manner that the qualita¬
tive effects and relationships of the ballistic
parameters are seen clearly. The resulting
simp? led equations are readily optimized for
determining minimum volume and minimum
weight rifles, and are used with little .'oss of
accuracy when compared to the more com¬
plicated solutions.
The general solution of the differential Eq.
5-54 is
x =K t exp(ip'V) - qV-^r+r’ (5-55)
With the initial condition that at x = x Q , V = 0
x ° = K i~$ + r '
5-18 METHOD
The equation of state, Eq. 5-35, can be
written as
^ = 4>'(x +qV~ r'), in. -(fps)' 1 (5-54)
where
Solving for K x and substituting into Eq. 5-55,
the specific solution becomes
x = \x 0 + ^jr- r'jexpty'7) - qV
in*
(5-56)
P, N, and N* have been eliminated by the
use of the following substitutions
*' “ (?j(t)tn’ aoc -"‘ l (5 - 54a)
Eq. 5-55 is an exact analytical solution of Eq.
5-52 provided that the correct value T 0 /T can
be determined. In Ref. 6 it is shown that with
some simplification of the basic energy equa¬
tion a good approximation of ( T 0 /T )* a is
5-47
Preceding page blank
AMCP 706-238
hi IMHKI wwaiwiwnww
"W W W8W.H .» —
AMCP 706-238
Required:
= 1 - 0.064 = 0.936 [Eq. 5-57bl
V m = 1120 fps
Find:
0 537
a = 1 - (2)K936) = 0/ ' 13 lE *- 5 ^ 7dl
(1 + 0.2X1.24 - 1K1.474H0.53)
2(6.46 x 10^X3.31 x 1C 5 )
The calculations that follow are made
according to the steps outlined in par. 5-18.
The value of W usually is chosen such that
the propellant wiil be “all burnt” about the
time the muzzle velocity is attained oi slightly
before. A longer burning time results in loss
of unbumt propellant, and a shorter time
causes excessive peak pressures given in the
same parameter set. The relation between the
web and muzzle velocity is then estimated as
W 0 = m'BVjA (5-60)
After a reasonable value of J 2 has been chosen
and a value of 0.5 assumed for X the web W
can be calculated from Eq. 5-60.
= 0.535 x lO* 4 sec-ft"
[Eq. 5-57fl
^ 1 (0.537)(1.24)(0.936)
2
- (0.535 x lO^Mmo) = 0.629
(Eq. 5-67c]
/tV A _ / o.629\ 1/2 _ 0.0644
\Tj ~ \0.713/ 2(0.713)
= 0. 894
[Eq. 5-57al
[Eq. 5-57)
093) 2 = 1.19
If
6 = 1 - (0.537)(1.093) = 0,413 [Eq. 5-49)
m
1.04
(1 - 0.5)10 *1
(3X32.2) J
= 1.095 slugs [Eq. 5-301
Ref. 6 shows that for V -0 the gas temperature
T equals the average temperature T and cor¬
respondingly
then by Eq. 5-60,
*o
1.095(6,5 x 10* 4 )(1120)
13.72
= 0.058 in.
The value of X is then calculated from its
definition
r-er
= u
0,537
0.936
0.426
Since propellant is assumed to be all burnt
at time of muzzle exit
_ kA t W 0 (6.46 x 10- 3 )(9.31X0.058)
A C 2 B (i0)(6.5 x 10" 4 )
= 0.537
b = i(l. 24 - 1X0.537) = 0.0644
[Eq. 5-57e]
v b = v m
and
, , _ (1.19X1.474X0.537)
lpm (0.413X3.31 x 10 5 )( 6 .46 x 10*8)
= 1.067 x io- s (fps)* 1 [Eq. 5-54a)
AMCP 70*238
----_
0 (0.936)*(0.420(3.31 x 10 s )(6.46 x IQ-*)
= 0.992 x io' s (fps)- 1 [Eq. 5-54a]
L n ~x m -x 0 = 140 - 50 = 90 in.
v c =A Xo = (13.72)(50) = 685 in?
il)'„ = i(0. 992 x 10- 3 ) + — Y 10 ~ > ) t
1/2
1120
= 1.055 x 10" s (fps)" 1
[Eq. 5-59]
(1.095X6.46 x
X [17.1 -28.6(0.413)]
= 3.445 x lO" 3 in. -sec-ft* 1 [Eq. 5-54d]
13.72
= 12.46 in.
3.445 x lpr 3
1.067 x 10" 3
12.46
X _ 3 a 445 x io* 3 (1120)
3.454 x 1Q- 3
1.067 x 10- 3
+ 12.46
= 140 in.
[Eq. 5-56]
Vp ~4>p = 1. 055 X l(r 3 “ 947 ‘ 9 fps
[Eq. 5-58a]
(12)(1.095)(947.9)
p (13.72)1.055 x lO- 3
445 x IQ- 3
x [( 50+3 di7T^- 12 - 46 )
x gl<05Sxl0~ 3 (947.9) 3.445 X 10~ 3 1~ 1
” 1. 055 x 10' 3 J
= 7993 psi [Eq. 5-58]
, (12)(1.095)(1120)
m (13.72)(1.067) x 10“ 3
[( 50 +
3.445 x IQ- 3
1. 067 x l(T 3
- 12.46)
x e
1.067xi0" s (1120) _ 3.445 x 10~ 3 |~
1. 067 x 10' “
r»>»
F 3 ]
= 7647 psi
[Eq. 5-58]
5-50
AMCPTO-23S
SECTION VI
ANALYTIC EQUATIONS FOR OPTIMIZING CERTAIN GUN PARAMETERS
5-20 THE LIGHTEST GUN FOR A
SPECIFIED MUZZLE ENERGY
In Section V, methods were presented for
calculating pressure-travel and velocity-travel
functions from a given set of ballistic param¬
eters. By variation of these ballistic param¬
eters, it is possible to calculate families of
gun design alternatives from which an op¬
timum configuration may be chosen. It is
desired to design the lightest gun of specified
caliber, projectile weight, and muzzle veloci¬
ty. Neglecting certain auxiliary equipment oi
approximately fixed weight for all calibers,
the weapon weight is primarily a function of
the recoilless rifle volume and peak pressure.
It is desirable to select a peak pressure and
determine the ballistic parameters that result
in the minimum volume weapon of specified
A, m, and V m .
In Section V, Eq. 5-56 showed that
exp W m V m ) - qV m
v?lue of x m , it is satisfactory to neglect small
quantities that vary slowly. Therefore, the fol¬
lowing simplifications are introduced.
( 1 ) q = 0
(2) 4>p = a^m where a 2 is an
undetermined constant
(3)
= function of V alone.
With these assumptions Eqs. 5-56 and 5-6lean
be combined as shown in Eq. 5-62
12 m' exp WJ
" PpAeaWj 2 + '
(5-62)
For a given bore area the weapon volume is
determined by Eq. 5-62 and for a given
muzzle velocity and peak pressure is a func¬
tion of ip' and C) alone.
As shown in Ref. 7, the vdue of \p' which
minimizes the recoilless rifle volume can be
determined from
in.
dx m _
dK
= 0
(5-63)
and from Eq. 5-58 and Eq. 5-58a
The condition for minimum volume as
determined from Eq. 5-62 is
(5-61)
where
•p'p is defined in Eq. 5-59.
For a specified bore area, the minimum
weight rifle corresponds to the minimum
vdue of x m . In determining the minimum
K = W m
(5-64)
In order to determine the charge associated
with the optimum value of 'p' m , it is neces¬
sary to determine a value of X for a given
\p ' m . The value of X is determined from the
following equation:
= /(X, V m ), dimensionless
(5-65)
5-51
AMCP 70*238
The function of / can be evaluated as
shown in Ref. 6 and, by plotting / as a
function of X, a family of curves is obtained
with V m as a parameter. For a given V m there
is only one value of J and X which satisfies Eq.
5-65. The family of curves then reduces to a
single curve of / as a function of X which
satisfies the optimum condition. This curve is
plotted in Fig. 5-18 where f x applies to
minimum volume guns. The value of C 2 can
then be determined, knowing the value of X,
as follows
C 2 = kA t W 0 /\h, lb (5-66)
The chamber volume Ax 0 then can be
determined from Eqs. 5-62, 5-64, and 5-65.
The complete method of solution for the
minimum volume recoilless rifle, which for a
specific bore area and chosen value of P p
corresponds to the minimum weight rifle for a
specified muzzle velocity, is as follows:
Given: A, m , V m , A n propellant
constants
Chosen: P p
Assumptions: (1) V b = V m
( 2 ) N 0 * 0
Solution
U> *m=2/Vm
(2) f(V m ,\) = 4>„kF/(A/A t )
(3) A determined from curve f\ of
Fig. 5-18.
(4) m = X. 04 + (1
At iMs point C { is estimated.
(5) B is determined from Fig. 5-17.
(6) W 0 = m'BVjA
(7) C‘ l =kA t W 0 /(\B)
If the estimate of C t in Step (4)
was poor, Steps 4 through 7
should be repeated. (Note:
C 2 = C { - C a )
(8) ipp = giiip'tJ determined from Fig. £-19.
/a v , T _ 12 mj
Xm Xo Lm PpA(>p'p)l
Co
x exp + —
U) m PpA{4>' p )*e
(11) v c =Ax 0
12 ttt' Co
< 12)
PM&y-sfi
(13) Pm~ rmexp(VmVm )
(14) A 0 = 27.7 C { /v c
It can be shown that for high muzziie
velocities the desirable minimum volume rifles
occur at high loading density. If the required
loading density is impractical, there are two
possible remedies. The loading density may be
reduced by using smaller values of X or
smaller values of V b . in the usual case, it
would be better to use smaller values of X
than to use smaller values of V b . In general,
however, some calculations for smaller V b
should be made to determine which pro¬
cedure for reducing the loading density yields
the smallest final weapon volume. For calcula¬
tions involving V < V b> the equations of Ref.
6 can be used up to V =V b . For V > V b an
“after burnt” solution such as given by
Hirschfclder (Ref. 8) must be used.
2.5 x 10
1. 5 x ID
0. 5 x 10
Figure 5-19. ^ as a Function of ^
AMCP 70*231
5-21 THE SHORTEST GUN FOR A
SPECIFIED MUZZLE VELOCITY
The barrel length can be obtained from
= ** ~ x 0
Substituting x m from Eq. 5-62, x 0 from Eq.
5-61 for the condition q * 0 into the above
obtain
The following gun and projectile values
have been chosen.
A = 12 in?
m =0.5 slug
A t = 8 in?
L - ’ ^ Sfe r!exp iKVJ ■ 11 (5 ■ ,i7,
The minimum barrel length is determined
when
Henc-
V M = 1500 fps
It is required to determine the barrel length
L m for minimum volume rifle. For the first
calculation, a peak pressure of 10,000 psi is
chosen
^- = 15M = 1 - 33X 1(rS <fl)8) ' 1
2[eXp4„l' M ) - .11 , „ rr \
-- - exp W m Vj = 0
$* (5-68)
The solution to Eq. 5-68 yields
The complete solution for the minimum
barrel length rifle is then the same as outlined
in the preceding section with the exception
that for a calculated /. the corresponding
value of X is determined from the / 2 curve as
shown in Fig. 5-18.
/(V,,,X) = (1.33 x 10 _J )(6.5 x i<r s )
x (3.3 x 10 5 )/(12/8) = 1.92
X = 0.58 (from Fig. 5-18)
Ci = 6 lb (estimated)
31 - 04 [ 0 - 5 * ^ Taf .’ a 16 ] = °- 546 alu «
B = 6.5 x lor 4 uMpsi-ecf 1
(from Fig. 5-17)
5-22 NUMERICAL EXAMPLE
The following is a demonstration of the
method of par. 5-20 applying the indicated
steps.
W.
12
C*
(6.5 x lO^HsHO. 044)
(0.58)(6.5 x lO" 4 )
= 6.06 lb
Given the following propellant values: ^ = 1.28 x 10* J (from Fig. 5-19)
rj = 28.6 inMb -1
= 12(0.546) (2,71828) 2
1/p = 17.1 in?-lb' 1 x * l r J>‘ l (12)(l.28 x 104)2(2.71828)
k = 6.5 x 10“* tiec* 1
+ (Note ■= 2)
F ®3.3 x 10 s (ft—’b) —lb" 1
= 90.6 + 8.6 = 99.2 in.
5-55
10 (12)(1.28 X 10r*)*(2.71828)
78.3 in.
12199.2 - 78.31 = 250.8 in*
12(0.546)
10 4 (12)(1.28x 10r*)i*
(6.06) (17.1) 250.8
+ 12 12
33.3 + 8.6 - 20.9 = 21.0 in.
■ 1. S3 x 10"*(1500)(2.71828)
= 6830 psi
The rifle weight can then be calculated by the
method of par. 5-35.
The process is repeated for several choices
of peak pressure and a plot of weapon weight
vs peak pressure results.
AMCP7CS»23S
SECTION VII
INTERIOR BALLISTIC SOLUTION USING DIGITAL COMPUTER
The digital computer is an excellent tool
for solving the interior ballistic equations
without *he need for additional simplifying
assumptions.
An actual computer program will no! be
presented in this handbook since it is simple
to write a program given a thorough under¬
standing of the physical principles. It is
important, however, that the user of the
program determine that the program indeed
lepresents his physical problem, or requires
modification.
A basic rationale employed in writing the
program follows:
(1) An initial shot-start pressure as dis¬
cussed in Chapter 11 is chosen.
(2) Over a short interval of time, e.g.,
0.0001 sec, the pressure in the gun is assumed
to be constant. If P x is the actual pressure at
the beginning of the time increment and
the pressure at the end of the time increment
then it is assumed that an average pressure F ~
(Pi + P 2 )/2 is constant over the time in¬
crement. The error of this assumption is
negligible over a sufficiently short time in¬
crement. The verity V t travel I, propellant
burnt N y and propellant gas accumulation N* %
based on the assumed average pressure are
then
^2 = fps
tfl
(5-69)
*2 =*i + V2)Mt in *
(5-70)
CjB _
*2 lb
(5-71)
0
fC.B /VV'2 1
*2 = *t + [-{£- " S\f) lb
(5-72)
where a value of T is obtained from solving
the c ncrgy equation, Eq. 5-47.
(3) The pressure P 2 ' at the end of the time
increment can then be calculated from the
equation of state, Eq. 5-35, as follow*:
Pi = 12F*£[a<* +L)
- ~ * p8i
(5-73)
The calculated average pressure P' = ( Pi +
P\)l 2) is then compared with the assumed
average pressure.
(4) If IF'_FI > 100 psi, for example,
repeat the calculation using the value of F 1 as
the assumed average pressure. After a few
iterations the assumed and calculated values
of average pressure will converge toward a
true value and
IP' -P l< 100 psi
At this point the calculation is sufficiently
accurate and the next time increment can be
calculated.
(5) For the first time increment, P x is
taken as the value of shot-start pressure with
x x = 0 and V x - 0. As a first approximation
in each increment, the assumed average pres-
suie can be taken as equal to the initial
pressure of the increment.
5-57
AMCP 706-238
(6) A running tabulation of all variables is
kept and, when the amount of propellant
humt equals the propellant cfcm-ge, the pro¬
gram switches to the governing equations
applicable to the “all burnt” condition.
Since the computer program only calcu¬
lates P(t), V(t), and x(t) for a given set of
weapon parameters, it is recommended that
approximate values of optimum gun para¬
meters first be calculated by the method of
Section VI. The parameters then can be used
as inputs to the digital computer for a more
accurate solution.
5-58
AM CP 706-231
SECTION VIII
SOLUTION FOR AFTER "ALL-BURNT' CONDITION
5-23 INTRODUCTION
In the previous discussions of the interior
ballistic equations, it had been assumed that
the propellant continued to burn to muzzle
exit so that a solution afte: “all-burnt” was
not required. This is often the case in the
design of a recoilless rifle since it results in a
high piezometric efficiency and therefore a
light gun.
However, it may be desirable to end propel¬
lant burning sooner than muzzle exit. When
the propellant charge is consumed before
muzzle exit, the interior ballistic equations
must be modified to reflect simple gas expan¬
sion. The projectile velocity coinciding with
end of burning is designated / b . Normally the
minimum weight weapon corresponds to V b =
V m , however, in practice, it is u:ually desir¬
able to end propellant burning before muzzle
exit to avoid excessive discharge of unburnt
propellant.
5-24 MODIFICATION OF EQUATIONS
FOR "ALL-BURNT" CONDITION
The basic equation for the rate of accumu¬
lation of gas in the rifle must be modified
after all-burnt. The weight of gas N* b in the
weapon at the end of burning is
N'„ = $ b C 2 , lb (5-74)
where
/T\l/2
6 b = 1 — X j , dimensionless
C 2 = propellant charge weight burned,
lb
rifle decreases as gas is discharged through the
nozzle. At this point, the equation for the
weight N* of the residual gas in the weapon is
N' = n ;- *(^) 1/2 (m' , lb (5-75)
The introduction of this equation for N 4
permits a solution for the pressure and
velocity.
5-25 SOLUTION OF EQUATIONS FOR
"ALL-BURNT' CONDITION
The step-by-step procedure prt..nied in
Section V can be continued for the case
where V> V b as follows:
The equations are
(1) N b /C 2 = 0, dimensionless
(2) 1' 2 = I(y — 1)X + 1] 1/2 , dimensionless
(3) g 0 -■ (1 — J^X/2, dimensionless
where J 0 = 0 for P 0 = 0
(4) g 2 = 1 — g a r 2 , dimensionless
(5) N&/C, =g t - g 0 /(T b /fJ 112 ,
dimensionless
where
( T \t/2
^■J = 1/ (T 0 /T ) 1 /2 for up to “all -burnt”
(6) <t>2=2(gi-^)/(H' b /C 2 ),
dimensionless
Thereafter, trie amount of gas in the recoilless (7) tp - l/d >2 + 1, dimensionless
5-59
AMCP 706-238
(8) > = 1 + (1 + p)(y — 1), dimensionless r 2 = [(1.24 — 1)(0.537) + 1] 1/2 - 1.062
<9) t! ‘ vcpAu - y> • dlme “ lonl<iSS
do) y' - |i + [l * (2 -
-t,[*-»-»£]/
*-{yJ }
(id * = r Xb
5-26 EXAMPLE
g 0 = 0.537/2 = 0.268
gi = 1 - (0.268X1.062) = 0.715
§*°- 716 'S =0 - 470
y = 1 + (1 + 0.2)(1.24 - 1) = 1.288
. 12(1.095)(1120) 2 ($)
“ 685(7641)01.043) 1 * 288 (2 - 1.24)
= 1.402
In par. 5-19, a numerical example was given
in which burning ended with a muzzle veloc¬
ity V m of 1120 fps. Assume that nothing is
changed in the previous example except that
the required muzzle velocity is 1200 fps.
Thus, at the end of burning
V„ = 1120 frs
x b = 140 in.
r = jl + [1 +(2 - 1.288X1.043)1(1.043/‘-“"-‘hi. 402)
(1.402)[l, 959 - (1.288 - 1) iM]) t ' l/<l,1 "' in
1.959 - J
= 1.406
x m = 1.406(140) = 197 in.
P b = 7641 psi
The problem is to calculate new values of x m
and P m as follows, using the steps outlined in
par. 5-25:
L m = x m — x 0 = 197 — 50 = 147 in.
= 4460 psi
120oV l 1,288
1120/J
5-60
t Mmw&qr f
wiww >www »r «w ■jjwji'..iwuu«u.iw
MM
AMCP 706-238
SECTION IX
HEAT TRANSFER
6-27 INTRODUCTION
The transport of thermal energy from
propellant gases to the bore and nozzle
surfaces of the recoilless rifle deserves serious
co.isideration. Heat transfer degrades interior
ballistic performance, increases the erosion of
nozzle and bore surfaces, may cause pre¬
mature ignition or chemical deterioration of a
round of ammunition, introduces difficulty in
the handling of a shoulder-fired recoilless
rifle, limits the maximum rate of fire and
finally, but not the least important, dimin¬
ishes the physical strength of the gun material
as the temperature of the barrel rises. For
example, in the latter consideration, the
strength of some gun steel alloys drops
approximately 18 percent for a rise in temper¬
ature from 70° to 500°F. This loss in strength
assumes even greater importance when one
considers that the recoilless rifle has a very
low overall heat capacity compared to the
conventional closed breech system; this fact is
reflected in a significantly higher and more
rapjd temperature rise.
length, and (3) convective heat transfer occurs
at the intericr and e::terior gun walls, the
basic equations for thin './alls and little
curvature describing the heat transfer in the
recoilless rifle are:
,,n*M ~ gg(r,j)
&r 2 J 8 t
(5-76)
with the boundary conditions
(1) - = h t W 0,f) - 6,1
(2) = VW>
and the initial condition that
d (w, 0) = 0
where
6(r,t) = T w (r,t ) - T a = wall tempera¬
ture above ambient, °F
The problem of estimating the temperature
distribution through the wall of the chamber
and barrel as a function of time is challenging.
The paragraphs that follow provide some of
the theoretical bases upon which a satisfac¬
tory method of temperature estimation is
obtained when combined with a minimum of
experiment".!; support.
5-28 BASIC EQUATIONS
T w (r,t ) = wall temperature at position r
and time /,°F
T a = ambient temperature, 0 F
r = outward radial distance into
wall, in.
t - time, sec
a' = diffusion constant, in?-sec- -1
Based on the assumptions that (1) heat is
transferred conductively only in the radial
dii action of the gun barrel, since the effects
of longitudinal and circumferential heat flow
are slight, (2) , : gun barrel is of infinite
k T = thermal conductivity of rifle,
Btu-(in?-sec-°F/in.)‘ 1
hj = heat transfer coefficient at in¬
terior wall, Btu-(in?-sec °F) -1
5-61
AMCP 706-238
h Q = heat transfer coefficient at ex¬
terior wall, Btu-(in?-sec. 0 F)‘ J
w = wall tliickness, in.
0 g = propellant gas temperature
above ambient, deg F.
5-29 SOLUTION OF THE EQUATIONS
The temperature of the recoilless rifle
now will be determined by two differ¬
ent methods of solving the basic equa¬
tions presented in par. 5-28. The first
method defines an equilibrium temperature
on a single shot basis, and then determines the
round necessary to achieve any particular
percentage of this equilibrium temperature at
a specific rate of fire. The secc nd method
deals with the determination of the weapon
temperature as a continuous function of the
number of rounds for a given rate of fire.
Before discussing either method of solu¬
tion, it is possible to write the basic equations
in dimensionless form by introducing the
following dimensionless quantities which are
referred to as reduced distance ?, reduced
time r, and reduced temperature <p:
4 = r/tv
r = t/t 0
<P = 6/
where 0 o = maximum temperature above
ambient at inner wall, °F, and t Q is defined as
t 0 = w 2 /a ', sec
Substitution of these quantities into Eq. 5-76
yields
= (5-77)
where
<p(i, r) = dimensionless
i)l
Since the ballistic cycle of a recoilless rifle
is approximately 12 msec, and the rate of fire
usually does not exceed 20 rounds per minute
(3 sec between rounds), the heat input to the
rifle can be defined as a delta “function”, i.e.,
a certain quantity of heat is instantaneously
transferred each time a round is fired. The
entire formal problem can then be described
as follows:
r > 0
H * (5-78)
(C-79)
(5-80)
</>(l,0) =0
(5-81)
where
rou,>d
Q 0 = heat flux input per round,
Btu-(in. 2 -round) -1
h” = hw/hj, dimensionless
h = heat transfer coefficient,
Btu-(ia. z -°F-sec)" 1
6[t — (n — I)tr] is the delta function
defined in such a manner that
f-
J i(x)dx = 1, and 6(x) = Oforx^O;
where in this case, x - t- (n - \)t r .
AMCP 70&-238
t r = dimensionless time between rounds
= «oRfT l
To determine the temperature of the exter¬
ior wall on the basis of pulsed heat input at
the inner boundary and convective heat loss
at the outer boundary, the dimensionless Eqs.
5-77 through 5-81 are solved through the use
of Laplace and inverse Laplace transforms as
outlined in Ref. 9. The final solution of the
reduced temperature at the exterior is written
as
Hir - (w - DteI exp {- /^'[t - (n - 1 )t p 1)
Mi
where
q a - heat transferred to the
weapon per round,
Btu-ronnd -1
Rf - rate of fire, rounds min' 1
q 0 Rf = heat transferred to weapon
per unit time, Btu-min -1
60 hA w d = heat loss by cooling per unit
time, Btu-min -1
d&
Rf c w ^w~dii = net ® ain h eat weapon
per ur.it time, Btu-min -1
(^-82) W w
wear on weight, lb
where
^(1, r) = reduced temperature at exterior
wall for reduced time t, dimen- A w
sionless
specific heat of weapon, Btu-
((5 weapon " 1
surface area of weapon being
heated, in?
H - Heaviside function which has the
following property
dO
dn
change in weapon tempera¬
ture per round, 0 F-round" 1
Hit -S) =
(o,t<s
U ,t*s
The solution to Eq. 5-83 may be written as
0(») = 0„[1 ~ exp (- nh„) 1, *F (5-84)
1/3/j interpreted as initial, reduced
temperature rise
Mi « (A" + l)/(2Q£), dimensionless
(5-82a)
In the determination of an equilibrium
temperature by the single shot analysis the
assumption of convective cooling at the
exterior surface still holds with the additional
assumption that the heat transfer coefficient
h is constant with change in temperature
From an energy balance, the following holds:
dO ,
RfC M) W w *£■ = q 0 R f - 60^0, Btu-min" 1
(5-83)
where
0, = = equilibrium temperature, °F
K =
60 hA^
RfC yjlf jj)
, round” 1
The initial temperature rise 0,, i.e., n ~ 1, is
then expressed as
di = 0(1) - 0(0) = 0 # [1 - exp (- V) (5-85)
Solving Eq. 5-85 for 6 e
6 .
0 = - * --
• 1 - exp (- hj
5-63
- t mw "V ***- ' •-w.’r'n"p>r«'»
|pry:
AMCP 706-238
The temperature decay 6 d after ths initial
temperature rise is given by
S d = exp IhAvt/lcvWj]
Therefore, tile temperature rise 0 l2 just
before firing the second round is
/ 60 A.Au, \
- exp (- V
Substituting into Eq. 5-85 and then Eq. 5-84,
the equilibrium temperature 0 e becomes
$. a 2
$ =- - — s -
* lmm e ]L o t -o a
6i
(5-86)
and the temperature, of the weapon at round
given by
-(Arl-Wl
(5-87)
5-30 TEMPERATURE DISTRIBUTION
DATA
5-30.1.1 Single-shot Analysis
The solution to the single-shot analysis has
been plotted in the form of nomograms as
shown in Figs. 5-20 and 5-21. Fig. 5-20 is a
nomogram of equilibrium temperature as a
function of initial temperature rise 0,, and
subsequent temperature decay 6 d until the
second round is fired. The quantities 6/0 e , R f ,
n, and h ' are paired, respectively, with an
indexing axis drawn between 0/d e and Rf
(Fig. 5-21). In order to determine 6/0 e -given
h “= 0.033, n =40, and Rf- 2-the following
procedure is used.
(1) Construct the straight line determined
by the points h ' and n, and extend the line to
intersect the index (Fig. 5-21).
(2) Construct the straight line determined
by the point of intersection on the index and
the point on the Rf axis determined by the
rate of fire. This line intersects the 0/6 e -axis
at the correct value (0.5).
The results obtained by using these results
from the single-shot analysis have proven
successful, being within the experimental er¬
rors shown in Table 5-4(A).
5-30.1.2 Determination of Temperature as a
Function of Round Number and
Rate of Fire
5-30.1 THEORETICAL CALCULATION
The solutions for both the single-shot
analysis, Eqs. 5-86 and 5-87, and the multiple
number of rounds for a given rate c fire, £q.
5-82, methods have been determined and
described in graphical form, Ref. 9, and are
presented in pats. 5-30.1.1 and 5-30.1.2.
Given the round number n, rate of fire Rp
cooling factor h\ the maximum temperature
at inner wall 7y, and the temperature of
inner wall after first firing T it the exterior
wall temperature easily is calculated by either
of the procedures outline l in pars. 5-30.1.1
and 5-30.1.2.
The solution for the exterior wall tempera¬
ture as found in Eq. 5-82 may be interpreted
as discussed in the paragraphs that follow.
After the initial temperature rise, the tem¬
perature begins its decay in such a manner
that when the second round is fired, its
contribution to the exterior surface tempera¬
ture is added to the residual effects of the
first round. In a similar manner, immediately
after firing the third round, the temperature is
calculated by adding the contribution of the
third round to the residual effects of the first
and second rounds. This process continues
until no further rise in the peak temperature
5-64
AMCC 706-238
Figure 5-20. equilibrium Temperature as a Function of initial
Temperature Rise and Decay
occurs, i.e., the equilibrium temperature has
been attained. In other words, the contribu¬
tion of the last round is exactly negated by
the decay of the preceding rounds.
The quantity 1/Afi may also be interpreted
as the initial, reduced temperature rise
1 0
"•‘‘I <5 - 88)
where 0 o is the maximum temperature rise a;
the inner wall. This quantity is independent
of the rate of fire and is a function of caliber
insofar as Q 0 and H are functions of caliber.
Therefore, the dimensionless quantity Mi is a
generalized quantity not only with respect to
temperature, but also with respect to caliber.
Figs. 5-22 through 5-31 are graphs cl MV vs
n for surface conditions h - 0.02 to 0.20 min”!
A typical calculation would be
Given:
T„ = 50 °F
T 0 = 500 °F
oz‘
AMCP 706-238
TABLE 5*4
COMPARISON OF THEORETICAL AND OBSERVED TEMPERATURE DATA
(A) Based on Single shot Analysis
Caliber, mm
57
57
105
105
Round No., n
1
2
3
4
5
6
7
8
g
10
11
12
13
14
15
16
17
18
19
20
(B) Comparison of Observed and Calculated
Results for the 57 mm, T6CE2 Recoilless
Rifle based on Determination of Tempera¬
ture as a Function of Round Number
and Rate of Fire
Temperature, °F
Calculated*
Observed**
Theoretical**
85
85
80
121
120
114
149
150
140
171
170
161
188
190
177
202
200
190
213
215
200
221
220
208
228
230
215
233
235
r? 19
237
240
*23
241
240
v 7
243
245
245
245
;
247
250
23
248
250
233
249
250
234
250
250
235
250
250
235
251
250
236
The initial temperature rise a'one is taken from empirical data.
Observed temperatures are recorded to within 5 deg F.
The initial temperature rise is calculated on the basis of Eg. 5-82a.
M lf ■
imm
t
MKTW2M
AM CP 7C6-238
Figure 5-25. Reduced Temperature vs Round Number for Given
Rate of Fire (h' » 0.08 min 1 )
5
10
15 20 25
30
35 40
Round Number n
Figure S-28. Reduced Temperature w Round Number for Given
Rate of Fire (h' * 0.14 min' 1 )
F
ffl
Figure 5-29. Reduced Temperature is Round Number for Given
Rate of Fire (h '« 0.16 min' 1 )
AMCP 706-238
Figure 5-30. Reduced Temperature vs Round Number for Given
Rate of Fire fh '« 0.18 min' 1 )
AMCP 706-238
Figure 5-31. Reduced Temperature Round Number for Given
Rate of Fire (h' = 0.20 min' 1 )
5-77
AMCP 706-238
where
T a
- air ambient temperature, °F
To
= maximum temperature at inner wall,
°F
r,
= temperature of inner wall after firing
first round,°F
Mi 1
_ -M__ o.078
0 o 450
From Fig. 5-30 for h '=0.18 min -1 with
Rf-2 and n = 15
Mx<P\s = 8.60
<P'u
II
V>15
= 8.60x 0.078 = 0.67
*15
= ">t$6 0 (remembering <p = d/6 0 )
*15
= 0.67 x 450
*15
= 302 deg F
As
= 0 15 + T a = 302 + 50
Ti 5
= 352 deg F
Table 5-4(B) also contains a comparison of
obsei /ed and calculated results for the 57 mm
T66E2 Recoilless Rifle for the following
conditions:
R f = 0.5 rd/min
b! = 0.12 min -1
T. = 85 °F
T a =41°F
T q = 500°F (assumed)
M~\ =• 0. 096
5-30.2 EXPERIMENTAL PHASE
Fig. 5-32 shows the experimental exterior
surface temperature distribution obtained
during firing tests of a 57 mm T66E2 Recoil¬
less Rifle. The rifle was outfitted with firing
stand and ten Chromel-Alumel thermocouples
of 40 gage wire spot welded into minute slots
along the barrel as indicated in Fig. 5-32.
These thermocouples measured the rifle
temperature as a function of time for various
rates of fire. In this case, as described in Ref.
10, the output from each thermocouple was
recorded on a multichannel galvanometer
type recorder. The recorder is calibrated by
substituting an equivalent resistance for the
thermocouple circuit, impressing on it a
known millivoltage, and iecording the deflec¬
tion obtained. Since the deflection of each
galvanometer is not r cacfl’ linear, the calibra¬
tion is accomplished in steps at approximately
1-in. intervals up to 4-in. total deflection tc
establish a calibration curve.
AMC° 706-238
SECTION X
SPECIAL TOPiCS
5-31 LOSS OF UNBURNT PROPELLANT
If unbumed propellant ejection through
the nozzle is W t at any time and the initial
charge is C,, the following relation exists:
C - N = C t - W a - N, lb
where
(5-89)
C = effective propellant charge weight,
lb
N = weight of propellant burnt, lb
From experimental investigations of the
powder loss from a 57 mm M18 Recoilless
Rifle as reported in Ref. 3, it was found that
the rifle at any time is
C - AT = C, - aim' V/A - N (5-90)
From the defini'.io' of uie propellant web
thickness W
W = W,
-*r¬
(5-91)
and from the approximation of a linear
burning rate r
r = C'P
the following expression is obtained for the
unbumt propellant web
Substituting P from the equation
of projectile motion and integrating up to
time of all burnt, the value of is found to
be
, 2 -1
“ = ii? “ SeC
where C s is the total unbumed propellant
charge ejected through the nozzle. The value
of W s at any time t is then
W s = a t i*Pdt = aim'V/A, lb
Jo
By substituting this result into Eq. 5-89, the
weight of unbumed propellant remaining in
W * W,
- 2C' f*j
Jo
or in dimensionless form
= x- 2 SL f
W 0 Jo
(5-92)
where
/= WIW 0 = fraction of web unbumt
Since the amount of unbumed propellant
ejected is proportional to the pressure
■ = aiP
then from Eq. 5-92 the amount of unbumt
Preceding page blank
AMCP 706-238
propellant ejected at any time t is
‘d zMjl 1
2Cf J
(5-93)
Therefore, the amount of unbumed propel¬
lant in the gun at any time is obtained by
using Eq. 5-89.
#
C-N~C { -N - a t W 0 { 1 -/)/(2C')
(5-94)
If we let .1' be the number of grains in the gun
at any time and n’ 0 the initial number of
grains, Eq. 5-94 can be written cs
n' =n' 0 - a 2 V; 0 (l - f)/(2C')
(5-95)
By knowing the fraction of propellant loss,
s = C s /Cj, the constant a 2 in Eq. 5-95 can be
determined from the following relation
(5-96)
wheie
represents the ratio of ejected charge volume
to the total initial charge volume, and £ 0 is
the initial length of the propellant grains. The
result of integration yields
”' = r^( i -£- 2s+2s/ )
1 ” 3
1 ~ J (5-97)
Since the surface area S g of single-perforated
grain at any time is given by
2C
= ~-[q + R’) - 2R’a -/)]
W 0 P
where
C 2 - C{ — C s
the total surface area S of the number of
grains in the rifle at any time t can be written
as the product of the number of grains in the
rifle and the surface area per grain. The result
X [(1 +R') — 2R / (1 -/)]
(5-98)
The burning rate equation may be written in
the following form
= -
2 \dt)
and substituting this value of r into
-i®
which upon substituting into Eq. 5-96 and
defining R' - W 0 /%
obtain
■ - pSr
N_ pW 0
(5-99)
5-82
AM CP 703-238
Substituting Eq. 5-98 into Eq. 5-99 results in
the following integral
x [1 +R’ — 2R *(1 —f)]df
The integration ox this equation yields a
cubic in / which is reduced to a quadratic
expression by the following substitution
f i * if 2 - if
From these operations, it follows that
where W/L is the charge regressiveness con¬
sidering propellant loss and is found to be
( 5 - 100 )
5-32 PRESSURE GRADIENT IN GUN
The complete derivation for the effective
projectile mass is found in par. 5-10. The
purpose of the effective projectile mass is to
compensate for the difference in the chamber
pressure and the pressure acting on the base
of the projectile and the additional pressure
drop caused by friction at projectile-barrel
interface. With the introduction of effective
projectile mass m' it is possible to write the
equation of projectile motion in terms of the
chamber pressure as follows
dV _ AP
dt rrl
5-33 FORM FACTOR FOR PROPELLANT
BURNING
In par. 5-12 the rate of propellant burning
was given as
<M 3C 2
dt W 0
and upon integrating
N _ Bm' V
C 2 AW 0
assuming that the burning surface of the
propellant is constant (single-perforated
grains) or as discussed in par. 5-12 for
seven-perforated grains
N _ Bm’V
C 2 AW 7 F 7
where the factor FV is shown in Fig. 5-33.
This approximation can be improved by
use of a form function that expresses the
fraction of propellant charge burned N/C 2 as
a quadratic function of the fraction of the
unbumed web f. The following function fits
accurately the common propellants in use
~ L = feo-*l/+*2/ 2 (5-101)
c 2
where ko, k\ , and k 2 are constants that
depend upon the propellant granulation.
The values of k 0 , k lt and k 2 are given in
the subsequent paragraph for the following
granulations with no unbumed propella.it loss
through nozzle injection.
5-83
AMC? 70*236
Maximum Pressure P^, psi
Figure 5-33. Multiplying Factor F 7 for Converting 7-perforated Webs
(VI-,) of M10 Propellant to Equivalent Single-perforated Webs (VI)
(1) Single-perforated Grain:
£-(•-5M9'’«-
(2) Seven-perforated Grain:
The form function of the seven-perforated
grain to splintering can be written in slightly
different form in terms of the fraction of
propellant web burnt.
+ *< 0) J + * 8 C0 S ( 5 . , 03)
where
u> -\-f - fraction of propellant web
burnt
The constants Ar*. k A , and k s are as follows:
i n 3 i)
X , o _
’ 1+ ©](
4 L ®Jj
.r,. _ 3 „im_ 3
ffi
_1
.‘©Jl
(5-104)
j
1
_3_r
\Pb)
d)
3 ,
(5-105)
1 )
32 L ©I
ju) 1
(5-106)
where
D 0 = initial diameter of grain
d - perforation diameter
£ 0 = initial length of grain
5-84
AMCP 706-238
(3) Cord and Sheet Propellants:
(a) Cord or Cylindrical Solid:
(b) Sheet Propellant:
~=1+/ (5-108)
15-34 MUZZLE FLASH
6-34.1 BASIC THEORY
When a recoilless rifle is fired, the hot gases
discharged from the muzzle and nozzle are
luminous, resulting in a visible flash. Since
flash reveals the location of the weapon, it is
an undesirable effect that should be mini¬
mized or eliminated. The flash resulting from
gases issuing from the nozzle of a recoilless
rifle is similar to muzzle flash, hut is of much
greater intensity because of the larger amount
of gases discharged. A study of flasn
phenomenon reveals three regions of lumi¬
nosity (Refs. 11,12, and 13):
(1) A small region of low luminosity at
muzzle or nozzle called primary flash.
(2) A region of high intensity just beyond
the muzzle or nozzle and separated from
primary flash known as intermediate flash.
(3) An ill-defined region of high intensity,
beyond but not well separated from the
intermediate flash, called the secondary flash.
Although the flash mechanisms are not
understood fully, it generally is agreed that
the propellant gas emerging from the gun is
sufficiently energetic to be self-luminous. The
gas immediately expands and cools below the
luminous temperature, resulting in a dark
zone. Following this, the gas is overexpanded
and is rec.impressed adiabatically through a
shock. This recompression raises the gas
temperature and the gas becomes luminous
again. The propellant gases have entrained air
during this process, thus forming a com¬
bustible mixture. If the recompression has
raised the mixture temperature to the ignition
point, it will form a secondary flash.
Most of the luminosity in fl^sh is due to
metallic impurities in the propellant gases.
The spectrum of the emitted gases reveals
strong radiation from sodium, potassium, and
calcium, and the oxides of calcium and
copper which are expected since these mate¬
rials arc used in propellant manufacture with
copper originating predominately from the
rotating bands.
5-34.2 FLASH SUPPRESSION
In principle, flash can be partially sup¬
pressed by the elimination of metallic im¬
purities. However, this approach is not
economically feasible. It has been determined
experimentally that flash can be partially
suppressed by the addition of propellant
additives or suppressed mechanically by the
insertion of flow spoilers.
Chemical suppressors inhibit combustion in
the secondary flash zone only. Many of the
additives that have been effective include the
following compounds of potassium: iodide,
bromide, oxalate, acid oxalate, and sulphate.
Mechanical methods have been more suc¬
cessful in suppressing flash. The general effect
of mechanical suppressors is the inliibition of
combustion since the gas dynamics are suit¬
ably perturbed. For example, when steel bars
are introduced into the gas stream, the flash is
reduced significantly.
Experimental studies also have been per¬
formed to determine the effect of gun design
on flash. This work indicates that flash
intensify is suppressed with reduction of peak
5-85
AM CP 706-238
i- 1 -
chamber pressure and increase in nozzle ex¬
pansion ratio. Flash intensity reductions of
50% were achieved by this means but only by
using a nozzle expansion ratio of seven which
is not usually optimum.
5-35 CALCULATION OF "BARE" GUN
WEIGHT
An important impact of interior ballistic
design is the effect upon the minimum
weighted gun. The calculation of “bare”
weapon weight suffices since ancillary equip¬
ment, gun sights, and spotting rifle, are
approximately the same weight for most gun
designs.
One proceeds to estimate weight con¬
sidering the recoilless rifle a cylindrical tube
of length x , cross-sectional area A \ »th wall
thickness w, required to sustain the internal
gas pressure.
The required wall thickness is
u>=P'R z / <7, in. (5-109)
where
where the factor 1.15 represents a 15 percent
' safety factor. The pressure in the gun is
estimated to be P‘ max for 0 , and
then to decrease linearly from P' to P b for
x p < x< x h and then with a linear decrease
from P b to P‘ n for x b < x < x m where
P' b = 1.15 P„
1. 15mgP m
MZ " (
V”* + ~)
Tire wall thicknesses w corresponding to these
pressures are determined as
\ xt> * x * x m
(MU)
P' - pressure at point in consideration,
psi
R 2 = radius of tube, in.
o = allowanle tensile strength of mate¬
rial, psi
/’'then is expressed in terms of P, the space
average pressure in the weapon. At the instant
P-Pp, any point of the tube up to x = x p
must withstand a maximum pressure P* max of
1M + *
C,0
cJ)
M *3
(5-110)
The tube weight is then given by
W t - 2irp'J« 2 + »<*)«(*
W t ^ , *' 2 1 —j (x b - x p )
+ ( s 4 J!k K-^]
+ + (t<‘p + U’ptv b + ui)
+ iwl + u> b t< „ + wl) <y P ~
(5-112)
5-86
i
I
AMCP 706-238
where p ' = density of gun mrterial.
In the case that V b is less than V p , P p = P b
and x b ~ x p which eliminates the second and
fifth terms of Eq. 5-112. In the case that
x b = x m , the third and sixth terms of Eq.
5-112 are eliminated.
5-36 LIST OF NUMERICAL CONSTANTS
USED IN INTERIOR BALLISTIC
CALCULATIONS
Table 5-5 is a list of the numerical values of
the constants for M10 Propellant and other
parameters used in the interior ballistic calcu ¬
lations.
TABLE 5-5
NUMERICAL CONSTANTS USED IN
INTERIOR BALLISTIC CALCULATIONS
For M10 Propellant:
V
1/p
To
F
r t
rt.
Other Parameter*:
K
A/A t
AJA t
28.55 in?-lb’ 1
17.09 in?-lb' 1
2869° K
3.31 x 10 s (ft-lbj-lb -1
4.53 x 10‘ 3 P 0, 'in.-sec I
1.24
6.46x Id 3 sec*
1.5
2-3
REFERENCES
1. AD 95182, D. J. Katsanis, A New Con¬
cept of the Ballistic Efficiency of Recoil-
iess Rifles, Frankford Arsenal Report
R-1312, March 1956, 9 pp.
2. Interior Ballistics of Recoilless Rifles,
Final Report ORD Project No. TS4-4004,
Armour Research Foundation of Illinois
Institute of Technology, Frankford
Arsenal, Philadelphia, Pa., April 1952, 3
Volumes.
3. D. J. Katsanis, A Graphical Method of
Solution of Intenor Ballistic Problems in
Conventional Recoilless Rifle Design, Re¬
port No. MR-604, Pitman-Dunn Labora¬
tories, Frankford Arsenal, Philadelphia,
Pa., May 1955, 18 pp.
4. S. G. Hughes, Summary of Interior
Ballistics Theory for Conventional Re¬
coilless Rifles, Report No. R-l 140, Pit-
man-Dunn Laboratories, Frankford
Arsenal, Philadelphia, Pa., September
1953,39 pp.
5. AD 36531, Development of 105mm Bat¬
talion Antitank Weapons and Interior
Ballistics for the Design of Recoilless
Rifles, Summary Report, Volume I, ORD
Project TS4-4020, Armour Research
Foundation of Illinois Institute of Tech¬
nology, July 1, 1954.
6. S. G. Hughes, Simplified Interior Ballistic
Equations for Recoilless Rifles with Zero
Starting Pressure, Report No. R-106i,
Pitman-Dunn Laboratories, Frankford
Aisenal, Philadelphia, Pa., April 1952, 12
pp.
7. S. G. Hughes, Conditions for Theoretical¬
ly Optimum Recoilless Rifles. Report No.
R-l 102, Pitman-Dunn Laboratories,
Frankford Arsenal, Pliiladelphia, Pa.,
October 1952, 13 pp.
8. J. O. Hirschfelder, R. B. Keishner, C. F.
Curtis, and R. E. Johnson, Interior
Ballistics of Recoilless Guns, OSRD Re¬
port No. 1801, NDRC Report No. A-215,
September 1943.
5-87
AM CP 706-233
9. AD 115524, H. Kahn, Temperature
Distribution in Recoilless Rifles, Report
No. R-1321, Frankford Arsenal, Phila¬
delphia. Pa., May 1956,29 pp.
10. AD 34245, Symposium of Recent
Progress of Recoilless Rifles and Am¬
munition, Department of the Army, Jan¬
uary 1954.
11. Rudolf Ladenburg, Report on Muzzle
Flash. BRL Report No. 426, 1943.
12. AD 224762, Rudolf Ladenburg, Studies
of Muzzle Flash and Its Suppression,
BRL Report No. 618,10 February 1947,
23 pp.
13. AM CP 706-255, Engineering Design
Handbook, Spectral Characteristics of
Muzzle Flash.
BIBLIOGRAPHY
AMCP 706-150, Engineering Design Hand¬
book, Interior Ballistics of Guns.
AD 73766, Research on Basic Studies of
Flash Characteristics of Recoilless Weapons,
Dept, of the Army, September 1955.
G. Seitz, The Influence of the Geometric
Forms of Powder Grains on Their Burning
Rates, (report translated by K. P. Gerhard
from Sprengtechnik No. 12, 1952 and
Explosivui-jffe No. 1/2), Picatinny Arsenal,
1953, 30 pp., Picatinny Arsenal Translation
No. 1.
Jerome M. Frankie and James R. Hudson,
Propellant Sv.rface Area Calculations for
Interior Ballistics Systems, AD 213-441, BRL
Memo Report 1187, January 1959, 33 pp.
J. Comer, Theory of the Interior Ballistics of
Guns, J. Wiley & Sons, New York, 1950.
T. J. Haves, Major General, Elements of
Ordnance, J. Wiley & Sons, New York, 1938.
AD 105-887, L. E. Stout and W. A. Dittrich,
Analog Computer Study of Interior Ballistic
Equations, Report No. R-1313, Frankford
Arsenal, March 1956, 24 pp.
AD 296-282, A. Magar, Burning Rate
Characteristics of M5 Propellant, Frankford
Arsenal, FA Report R-1642, June 1962, 27
pp.
AD 265-123, J. Harris Shulman, and C.
Lenchitz, Burning Characteristics of Stan¬
dard Gun Propellants at Low Temperatures
(2J°C to -52°C), Picatinny Arsenal, Novem¬
ber 1961, 101 pp.
A. Magar, Burning Rate Characteristics of T18
and M6 Propellants, Frankford Arsenal,
October 1958.
CPIA/M2 Solid Propellant Manual, Revised
Edition, October 1965.
Sorrow, P.ccherches Theoriques Sur le Charge-
ment des Bouches a Few (Paris, 1882).
AD 229-048, D. J. Katsanis, A Theoretical
Interior Ballistic Study of Recoilless High-
Low Pressure Guns, Report R-1513, Frank¬
ford Arsenal, June 1959,47 pp.
W. A. Dittrich, Ninth Tripartite AXP
Conference, Paper A.2(a)l, 1958.
J. Mar, (S), A Feasibility Study of the
Internal Ballistics of a New Medium Anti-
Tank Recoilless Gun (U), CARDE Tech
Memo 292164, March 1960.
Hypervelocity Guns and the Control of Gun
Erosion, Summary Technical Report of
Division 1, NDRO, Volume 1, 1946.
C- L. Anni, et al., Measurement of Heat Input
to the Bore Surface of Caliber .50 Gun
S4J8
AMD* 706438
Barrels, OSRD 6470, Report A-399, Leeds &
Northrop Co., July 23,1945.
J. O. Hirschfelder et al., Heat Conduction,
Gas Flow, and Heat Transfer in Guns, OSRD
863, Progress Report A-87, Geophysical Lab.,
Carnegie Institute of Washington, July 1943.
G. S. Fulcher, Ed., The Temperature of the
Bore Surface of Guns, OSRD 1966, Report
No. A-201, Geophysical Lab., Carnegie
Institute of Washington, July 1943.
J. O. Hirschfelder et al., Interior Ballistics,
Part I, OSRD 1236, Report No. A-142,
Geophysical Lab., Carnegie Institute of
Washington, February 1943.
E. P.Hicks, C. K. Thornhill, The Heating of
Gun Barrel by the Propellant Gases, Report
No. 507-1, Watertown Arsenal, December
1942.
Heat Transfer in a 57 mm Recoilless Rifl <*
Based Upon Measured Internal Surface
Temperature, Phase Report No. 3, Midwest
Research Institute, Contract DA-23-072-
ORD-637, October 1954.
AD 404467, Bannister et al., Heat Transfer,
Barrel Temperature and Thermal Strains in
Guns, Report No. 1192, Ballistic Research
Labs., February 1963, 59 pp.
AMCP 706-107, Engineering Design Hand¬
book, Elements of Armament Engineering,
Part Two, Ballistics.
AD 73766, Research on Basic Studies of Flash
Characteristics of Recoilless Weapons, Depart¬
ment of the Army, September 1955.
AD 801-763, Douglas C. Vest, An Experimen¬
tal Traveling Charge Gun, BRL Report No.
773, October 1951,72 pp.
Fast Burning Propellant, Final Report, Con¬
tract DA-23-072-ORD-369, Phases I, II and
IV, Olin Mathieson Chemical Corp., East
Alton, Illinois, 1955.
AD 36566, D. C. Vest et al., A Qualitative
Discussion of the Burning Mechanism of
Porous Propellants, BRL Report No. 902,
April 1954, 30 pp.
AD 250-053, Paul G. Baer and Kenneth R.
Bryson, Design Data for the Constant Pressure
Traveling Charge Gun, BRL Tech Note No.
1360, November 1960,23 pp.
(C) Hypervelocity Weapons Feasibility Study
(U), Final Report, Contract AF08(635)-3543,
Illinois Institute of Technology, No. 1965.
CARDE 316/59(C), launcher Rocket, ATK,
100 mm Xc-1 (U), May 1960.
O. E. Teichman, Investigation of Temperature
Distribution and Powder Gas Flow in
Recoilless Rifles, Summary Report, Contract
W-l 1-022-ORD-l 1171, Armour Research
Foundation, Jan. 1949.
Development 2.76 in. Rtcoilless Rifles T-190
and T-l 91 for Mounting on Aircraft, Final
Report, Vol. I, Contract DA-11-022-ORD-
865, Armour Research Foundation.
J. N. Kapur, The Internal Ballistics of a
Recoilless High-Low Gun. Appl. Science
Research, Section A, Vol. 6, No. 5 •6.
S. P. Carfagno, (C) Handbook on Gun
Flash(U), Prepared for Ammunition Branch,
Office of Chief of Ordnance, U S Army, The
Franklin Institute, Philadelphia, Pa., 1961.
AMCP 706-255, Engineering Design Hand¬
book, Spectral Characteristics of Muzzle
Flash.
AD 467617, A. G. Edwards, Interior Ballistic
Analysis of Various Guns and Launcher
Systems, Picatinny Arsenal, TR-3193, June
1965,88 pp.
AMCP 706-238
AD 201104, Calculation of Interior Ballistics
of Recoilless Guns by Analog Computer,
Picatinny Arsenal, TR-2541, November 1958,
31 pp.
Recoiliess Rifle Handbook (Unpublished),
Prepared at Fiankford Arsenal, Philadelphia,
Pa.
5-90
fMCP 706-238
CHAPTER 6
CANCELLATION OF RECOIL
6-0 LIST OF SYMBOLS c p
A
= cross-sectional area of nozzle at
arbitrary ’ocation, ft 2
A b
= cross-sectional area of bore of
rifle, ft 2
c v
A c
= cross-sectional area of chamber
of rifle, ft 2
A.A.0,
A e
= cross-sectional area of nozzle at
exit, ft 2
F
A i
= cross-sectional area of nozzle at
. inlet (also called nozzle
approach area, or nozzle en¬
Fr
trance area), ft 2
G
A o
= cross-sectional area of nozzle at
reference location, ft 2
G'
A r
= cross-sectional area of nozzle at
throat, ft 2
G 'a
AH
a
= acoustic velocity at arbitrary
section of nozzle, ft-sec” 1
h, hi. h 2
a t
= acoustic velocity at throat sec¬
tion of nozzle, ft-sec" 1
b
= subscript which refers to bore of
rifle
k
C F
= thrust coefficient, dimensionless
M'
C
= specific heat of nozzle material,
caHg^QT 1
M = v/a
c
= subscript which refers to cham¬
ber of rifle
M e
specific heat of propellant gas at
constant pressure,
(ft-lbMlb^Fr 1
specific heat of propellant gac at
constant volume,
(ft-lbMlb^F)" 1
bore diameter of rifle, in. or mm
throat diameter of nozzle, in.
thrust force of nozzle, lb
recoil force (rearward) of rifle,
lb
mass velocity, slug-(ft 2 secf 1
mass flux (mass flow, or mass
flow rate), slug-sec" 1
ac* jal mass flow rate, slug-sec" 1
theoretical specific enthalpy
change, (ft-lb^slug" 1
heat transfer coefficient from
propellant gas to nozzle surface,
cal-(cm 2 -sec-°CF l
thermal conductivity of nozzle
material, caKcu^-sec^C/cmy 1
molecular weight of propellant
gas, dimensionless
Mach number at arbitrary lo¬
cation of nozzle, dimensionless
jet Mach number, dimensionless
6-1
AMCP 706-238
%MP
=
percent of melting point
P
=
pressure at arbitrary location of
nozzle, lb-ff 2
Pa
=
ambient pressure external to
rifle, lb-ff 2
Pc
=
chamber pressure of rifle, Ib-ft " 2
Pc
=
pressure at exit section of
nozzle, lb-ff 2
Pi
pressure at inlet section of
nozzle, lb-ff 2
Po
=
ideal reservoir pressure (pressure
at reference location), lb-ff " 2
P,
=
pressure at throat section of
nozzle, lb-ff 2
P t IP.
=
jet pressure ratio, dimensionless
PoIPe
=
pressure ratio, dimensionless
R
gas constant (= 19,709 /jW'),
ft 2 -(sec 2 -°RF l
r
=
radius oi exit section of nozzle
(jet radius;, in.
r'=A c /A t - ratio of chamber area to rifle
bore area, dimensionless
AT„ = difference between melting
Ttl
temperature of nozzle material
and initial temperature of noz¬
zle, °R
T 0 = ideal reservoir temperature of
propellant gas (temperature at
reference location), °R
AT t = temperature rise of inner surface
of nozzle throat (with reference
to nozzle initial temperature),
°R
AT, — temperature rise of propellant
gas al nozzle throat (with
reference to nozzle initial tem¬
perature), °R
t = time, sec
t - subscript which re Vs to throat
of noz7.<e
: m = time for projectile to leave
muz^.w uf rifle, sec
v = gas velocity at arbitrary location
of nozzle, fpi
v e - gas velocity at exit section of
nozzle, fps
v, - gas velocity at inlet section of
nozzle, fps
v, = gas velocity at throat section of
nozzle, fps
vjv, = velocity ratio, dimensionless
x - abscissa of a point on jet
boundary
y = ordinate of a point on jet
boundary
a = divergence angle of nozzle (an¬
gle of inclination of diverging
nozzle wall to nozzle axis), deg
2a = cone angle of nozzle (nozzle
expansion angle), deg
P = angle of inclination of converg¬
ing plug wall to nozzle axis, deg
7 -c/c v = ratio of specific heats of propel¬
lant gas, dimensionless
t = expansion ratio of nozzle, di¬
mensionless
AMCP 706-238
«i.«i
V
*KE
Vy
= fractional increase in nozzle X
throat area, dimensionless
t
= overall efficiency of nozzle, p
dimensionless
P
= discharge correction factor of 0
nozzle, dimensionless
P,
= kinetic energy efficiency of
nozzle, dimensionless
= velocity coefficient of nozzle,
dimensionless
= divergence correction factor,
mensioniess
di
* density of nozzle material,
g-cm" 3
= density of propeliant gas in
reservoir, slug-ff 3
= density of propellant gas at
nozzle throat, slug-ff 3
= dimensionless recoil (momen¬
tum ratio parameter), dimen¬
sionless
AMCP 706-238
SECTION I
INTRODUCTION
6-1 CONSERVATION OF MOMENTUM
The so-called “Recoilless Principle” is
derived from the general “Momentum Theo¬
rem” for a system of particles. This theorem
states that the time rate of change of the
momentum of the system is equal to the sum
of all the external forces acting cn the system.
Accordingly, in the absence of external
forces, the momentum of the system
undergoes no change. This result is known
popularly as the principle of “Conservation of
Momentum”. When applied to a gun discharg¬
ing a projectile forward, this principle shows
that the gun will itself be driven in motion in
a direction opposite to that of the projectile,
a phenomenon well known as recoil. In the
event that the gun is to experience no recoil,
it is evident that an equivalent amount of
rearward momentum must be generated by
use of a scheme in which the recoiling mass is
something other than the gun and, conse¬
quently, the recoilless guns do not, in general,
have the breech closed as in the case of
conventional guns.
Among the various schemes proposed, one
of the first was a gun that used a single
straight tube to simultaneously fire projectiles
of equal mass from both ends. A similarly
awkward scheme was employed in the Davis
gun that ejected simultaneously a projectile
with high velocity from the muzzle and a
heavy lump of lead with low velocity from
the breech of the weapon.
A simpler, though somewhat less obvious,
way of obtaining the recoilless effect is to
permit the transfer of a large portion of the
propellant gas to the rear, so that the
rearward momentum of the gas escaping from
the breech is employed to balance the
forward momentum of the projectile. A
constriction of the gun tube at a point behind
the propellant charge was employed in the
Cooke recoilless gun. The constriction of the
rear passage increases the velocity of the
escaping gas, raises the internal pressure, and
also increases the muzzle velocity of the
projectile. Recoillessness then becomes a
function of the ratio of the bore area to
exit-port area, ratio of the projectile mass to
charge mass, loading density, powder granula¬
tion and compositions, burning temperature,
and other factors.
6-2 THE SUPERSONIC NOZZLE
In actual practice, the constriction in the
rear passage is realized by having the
propellant chamber open into a rearward
orifice of a cross section somewhat smaller
than the bore and then into a divergent
nozzle. The convergent-divergent (de L£,vaJ)
nozzle thus formed allows the passage of a
large portion of the propellant gas and its
associated momentum to the rear. Upon
ignition of the charge, the propellant gases are
generated at a greater rate than can be
maintained in the efflur from the nozzle. The
rising pressure in the chamber then exerts a
force on the projectile to set it into motion,
and finally establishes a state of approximate
pressure equilibrium in the chamber during
the remainder of propellant combustion. In
this process the rearward momentum acquired
by the gas escaping through the nozzle is
controlled by the size of the nozzle throat
area and the expansion ratio (defined by Fxj.
6-5
Preceding page blank
AMCP 706-238
6-1) of the nozzle. By proper selection of
these quantities, the forward thrust exerted
by the reaction of the escaping gas on the
chamber and nozzle walls can be used to
neutralize the rearward thrust (recoil) which
otherwise would be communicated to the gun
by the reaction of the gas pressure driving the
projectile forward. Ideally, the nozzle will
maintain the weapon motionless during the
firing cycle. However, perfection is not sought
since slight variations in performance from
round to round are unavoidable. In the long
run, the progressive wear in the tube and
nozzle will aggravate these variations, and a
slight amount of recoil is tolerated. In fact,
some initial recoil is deliberately planned in
the rearward direction at as high a level as can
be tolerated, since this is desirable for longer
nozzle life. As the nozzle throat wears,
rearward recoil diminishes to zero and
ultimately becomes regenerated in the for¬
ward direction. Newly designed nozzles
should, therefore, have the throat restricted
more than necessary in the test gun to assure
a substantial rearward recoil.
Thus, recoilless weapons of given nozzle
design and geometry, to a certain extent, can
be adjusted to the desired recoil balance by
increasing the throat area to decrease
rearward recoil, or by decreasing the nozzle
length to increase rearward recoil. Increasing
the throat area is simpler than decreasing the
nozzle length and is most commonly used.
To recapitulate, the forward momentum of
the projectile, together with tuat of the small
amount of the propellant gases accompanying
it, is to remain practically equal to the
rearward momentum of the greater fraction
of the gases that issue from the nozzle in
order for the rifle to be recoilless. Strictly
speaking, however, impulses due to such small
factors as projectile friction and engraving, gas
drag, and the resistance of any diaphragm
initially restraining the exit of the propellant
gases from the nozzle must be compensated,
along with the elimination of the rotary recoil
(torque neutralization) of the rifle which
results from the helical motion of the
spin-stabilized projectile inside the gun tube.
For a given amount of recoil to be
eliminated, less mass of propellant gas is
required by use of high-velocity gas. The
convergent-divergent nozzle can produce
supersonic gas velocity. When the flow
completely Ms the nozzle, exit pressures
below the critical level (defined by Eq. 6-6)
cannot exist in a convergent nozzle, and the
exit velocity can never exceed the sonic value.
In a convergent-divergent nozzle, however,
the pressure along the nozzle can be less than
the critical value at any point past the throat,
and supersonic flow results in the dive*gent
portion of the nozzle-provided that the ratio
of the chamber pressure to the exit pressure is
sufficiently high to induce supersonic fiow-
since the exhaust velocity of the nozzle is a
function (see Eq. 6-14) of this pressure ratio.
Very high gas exhaust velocities can be
obtained in this type of nozzle, the increase in
the kinetic energy of the gas being derived
from a corresponding decrease in the
temperature.
Additional information on nozzle theory is
contained in par. 6-7 dealing with nozzle
design.
6-3 EFFECT ON INTERIOR BALLISTICS
The efficiency of a nozzle is based upon
the thrust it can produce. The thrust F is
proportional to the thrust coefficient C F (see
Eq. 6-20) which is a function of the
expansion ratio e as shown in Figs. 6-4 and
6-5. A large expansion rati? nozzle is highly
efficient and permits the use of a smaller
throat area for a given recoil balance and,
consequently, requires a smaller amount of
propellant charge. On the other hand, a large
expansion ratio nozzle is larger and heavier
than a low expansion ratio nozzle to the
extent that expansion ratios greater than 3
have not been worth the increased weight
they incur. Fast practice has been to use
nozzle expansion ratios between about 2.0
i
6-6
AMCP 706-238
and 3.0. In general, a value of about 2.S
represents a good compromise between
efficiency and weight. For this value of the
expansion ratio, the ratio of the area of the
bore to that of the throat required for
recoillessness is about 1.4S (refer to par.
6-11). For rccoilless rifles, the ratio of the
area of the bore to the area of the throat
varies, in general, between approximately 4/3
and 3/2. This corresponds to a nozzle
approach-area to rifl* bore-area ratio of
approximately 1.5 or greater.
I
i
i
I
i
AMCP 706-238
SECTION II
THEORY OF THE DE LAVAL
(CONVERGENT-DIVERGENT) NOZZLE
6-4 ASSUMPTIONS
The de Laval nozzle is of the “convergent-
divergent” type and has axial symmetry. In
the simplified, one-dimensional (hydraulic)
theory of nozzles, the following assumptions
are made:
1. The propellant g^ is homogeneous
throughout the chamber and nozzle.
2. The propellant gas is ideal, i.e., it obeys
the perfect gas law.
3. The gas has no viscosity.
4. There is no heat transfer across the
walls.
5. The gas flow is steady and irrotational,
with no shock, discontinuity, or separation.
6 . The flow is axially symmetric, the
velocity being axially directed.
7. The velocity and pressure of the gas are
uniform across any circular cross section of
the nozzle.
8 . Chemical equilibrium prevails.
Based on these simplifying assumptions, the
calculated ideal values of performance are
usually within 1 to 10 percent of the
measured values, so that the theory still gives
good prediction of results.
6*5 DEFINITIONS
Fig. 6-1 is a schematic of the de Laval
nozzle showing various design parameters.
The propellant gas enters the nozzle at the
inlet section i.
The minimum nozzle area is called the
throat area. The section at which the gas
leaves the nozzle to enter the surrounding
atmosphere is known as the exit e, and the
associated area of cross section is called the
exit area. The nozzle area expansion ratio e
or, simply, the expansion ratio is defined as
e -AjA t , dimensionless (6-1)
where
A e = nozzle exit area, ft 2
A t = nozzle throat area, ft 2
and the reciprocal of e may be calculated by
Eq. 6-12. The pressure at the throat for which
the gas flow per unit area of the throat is a
maximum is called the critical pressure.' The
ratio of the critical pressure to the pressure at
the inlet (or, more precisely, the constant
pressure p 0 of a large reservoir from which
the flow in the nozzle cjuld have arisen by
purely isentrepic flow) is called the critical
pressure ratio. This ratio is a function of 7
where
7 = c p /c v = ratio of specific heats, dimen¬
sionless ( 6 - 2 )
c p = specific heat of propellant gas
at constant pressure,
(ft-lb)-( Ib-°FJ~ l
c v = specific heat of propellant gas
at constant volume,
(ft-lbX lb-°F )“ 1
6-9
Preceding page blank
AMCP 706-238
inlet throat exit
Figure 6-1. Schematic of Nozzle Showing Design Parameter
6-6 BASIC EQUATIONS
6-6.1 RATE OF FLOW
From the assumptions of par. 6-4, it
follows that isentropic expansion relations
can be used in the de Laval nozzle flow. By
use of the energy equation, the equation of
continuity, and the assumption of the
isentropic flow of a perfect gas, it can be
shown (Ref. 1, p. 120} that the velocity v and
the pressure p at any section are related by
"7 Hg§T
(6-3)
where
A = cross-sectional area of nozzle at
location under consideration, ft 2
p - gas pressure at A, lb-ft -2
v = gas velocity at A , fps
P. = gas constant ( = 49,109/M'),
ftMsec-^RT 1
M' = molecular weight of gas, dimension¬
less
7 = ratio of specific heats, dimensionless
A Q - cross-sectional area of nozzle at a
reference location, ft 2
p 0 = gas pressure at A 0 , lb-ff 2
T 0 - gas temperature at A 0 , °R
Selecting the conditions of the reference
location to be those of an infinite reservoir
maintaining the isentropic flow in the nozzle,
one obtains from Eq. 6-3 the simpler
expression
-fen-fen--
(6-4)
where p 0 and T 0 now refer, respectively, to
the pressure and the temperature of the gas in
the inlet reservoir. In Eq. 6-4, p is the only
variable and it, in turn, governs Che magnitude
of the velocity.
6-10
AMCP 708-238
The mass fk>w rate per unit area A of the
nozzle (or, mass velocity G) is obtained from
Eq. 64, the equation of continuity, and the
perfect gas law, as
BluZ-^-aec)- 1
(6-5)
wh^re
Since yfyl FT' * a, represents the acoustic
velocity at the throat, Eq. 6-9 indicates that
the critical velocity (throat velocity) v f is
always equal to the local acoustic velocity in
the ideal nozzle in which critical conditions
prevail. The Mach number v f /a, at the throat
of the ideal nozzle is, therefore, unity
provided that the critical pressure exists at the
throat. It will be seen shortly (see Eq. 6-14)
that if the exit pressure p t is such that
/*• / 2 V /<rol>
Po ~ (—)
G * mass velocity, slugKft 1 -*ccr‘
G' * constant rate of flow of mass of gas
(mass flux), slug-sec' 1
p 0 * density of propellant gas in reservoir,
slug-ff 3
The value of the pressure ratio p t lp 0 for
which the mass velocity G is a maximum,
occurs at the throat and is obtained from Eq.
6-5 as
p t / 2
— = (--) , dimensionless
po \y + y
(6-6)
This is known as the so-called critical pressure
ratio and depends on the speciflc heat ratio y.
The density p, and the temperature T, at the
throat are given by
then, the divergent portion of the nozzle
permits a further decrease in pressure below
that at the throat and a corresponding
increase in velocity above the sonic throat
velocity; supersonic flow results in the
divergent portion of the nozzle.
Theoretically, the maximum possible value
of the nozzle exhaust (exit) velocity
is reached by letting p t lp 0 -* 0 in Eq. 64, i.e..
This corresponds, for example, to the case of
the nozzle exhausting into a vacuum. The
maximum value is never achieved in actual
practice because the temperature falls below
the point of liquefaction during the expansion
of the gas.
£■=2/(7 +D
1 0
(6-7)
( 6 - 8 )
The gas velocity at the throat v t is obtained
from Eq. 64, with p * p, t .".nd Eq. 6-6 as
v t =
RTg = 'JyRTf
(6-9)
It is seen from Eq. 64 that the exhaust
velocity v t of the nozzle is a function of the
pressure ratio p 0 lp e and the specific heat
ratio y of the propellant gas. Furthermore, v e
is proportional to the square root of the gas
constant R and the square root of the
absolute temperature of the ideal reservoir.
The gas constant R is inversely proportional
to the molecular weight of the gas. The exit
velocity v e increases with the pressure ratio
p 0 lp e and decreases slightly with the speciflc
heat ratio y. The influence of either of these
two factors on v t (or v, in general) is.
6-11
AMCP 708-238
however, less pronounced than that of the
absolute combustion temperature divided by
the molecular weight of the gas.
The area ratio for the divergent section of a
supersonic nozzle can be expressed, by use of
Eqs. 6-5 and 6 - 6 , as a function of the pressure
ratio and the specific heat ratio, as
( 6 - 11 )
where A is the downstream area where *he
pressure is p. At the exit, A = A e and p s p e ,
so that the expansion ratio e defined by Eq.
6 -« is giver by
a result already cited previously, following
Eq. 6-9.
The expansion ratio e given by Eq. 6-12
together with the velocity ratio of Eq. 6-14
are tabulated in Table 6-1 versus the presence
ratio p 0 /p e for 7 ■ 1.23. Note that the
v elocity rat io has the finite value of
y/ (7 + I JRy - 1 )' in the event of the
exhausting into a vacuum, while both the
expansion ratio e and the pressure ratio p 0 lp e
are infinitely large.
6-6.2 MASS FLOW
The rate of flow of mass of gas O' is
obtained, by considering the flow through the
throat area, as
G' ~A t v t p t , slug-sec ' 1 (6-16)
From Eqs. 6-16, 6-7, 6-9, and the equation of
state of the perfect gas, it follows that the
mass flow through the critical section of a
supersonic nozzle is
dn
(3-12)
The ratio of the velocity v at any point
downstream of the throat with pressure p to
the velocity v, at the throat follows from Eqs.
6-4 and 6-9 as
and the velocity ratio vjv { is
r-7=yp5RSn —
In a supersonic nozzle, v t > v, = >/ 7 RT t '.
From this and Eq. 6-14, it follows that
p. /_2_y/<>-n
Po \Y + 1/
(6-15)
G‘
L
4
y~)
slug-sec * 4 (6-17)
Eq. 6-17 shows that the mass flow through
the de Laval nozzle is proportional to the
throat area A ft the reservoir pressure p 0 ;
inversely proportional to the square root of
the absolute temperature o f the reservoir, and
is a function of the properties of the gas. Note
that Eq. 6-17 is independent of the exit
pressure p t , provided the latter remains below
its critical value of p Q ( 2 / 7 + 1 ) (7/t "* l> . In
other words-provided that the flow in the
divergent portion of the nozzle is super¬
sonic-lowering the exit pressure will not
increase the throat velocity or the mass flow,
and Eq. 6-17 represents the maximum value
of the mass ilow. In fact, this maximum value
is obtained more directly by use of Eq. 6 -S in
which A = A t and p - p n and Eq. 6 - 6 ,
together with the equation of state of the
perfect gas.
6-12
yvr^WfWI^r
<m
«*■
AMCP 706-238
TABLE 6-1
VELOCITY RATIO AND EXPANSION RATIO AS
FUNCTIONS OF PRESSURE RATIO (y ■ 1.23)
7- 1.23
frasuira
Velocity
Expansion
Ratio,
Ratio.
Ratio,
vW
e-A 9 /A t
2
1.085666
1.008103
4
1.487956
1.282250
6
1.661412
1.600248
8
1.767347
1.911414
10
1.841766
2.198208
20
2.0&213
3.4119333
30
2.136047
4.631886
40
2.198073
5.687256
50
2.242835
6.682456
80
2.277429
7.632444
80
2.328697
9.431303
100
2.365893
11.129566
oo
3.113784
oo
6-6.3 THRUST GENERATED PY NOZZLE
When a fluid in a duct experiences a change
in momentum, Thrust is said to be developed,
r the axisymmetric and unidirectional flow
ough the nozzle of Fig. 6-2, the thrust is
axial and its magnitude is equal to the surface
integral of the pressure forces acting on the
nozzle in the x-direction. In the event of
steady flow, the magnitude of the thrust force
can be shown to be (Ref. 2, p. 54)
F = G’v 4 +A e (p g - p a ), lb (6-18)
where p„ is the ambient external pressure.
The thrust F, which is the external force
acting on the nozzle, is seen to be the sum of
two terms. The first, which is the product of
the mass flow rate G 1 and the exit velocity v e
relative to the nozzle, is called the momentum
thrust. The second, which is the product of
the cross-sectional area of the exit of the
nozzle and the difference between the jet exit
pressure and the amb'ent pressure, is called
the pressure thrust. Since G’ - A.v t p ,, Eq.
6-18 may be rewritten in the general form of
F =A t p t v t v g +A g {pg — £a), lb
For constant value of y during nozzle
expansion, Eq. 6-18 is rewritten by use of
Eqs. 6-17,6-14, and 6-9 as
."[.-fen
+ A 0 [p t —p t ), lb (6—18)
Figure 6-2. Distribution of Forces Acting on Nozzle
AMCP 70&238
It is seen from £(|. 6-19 that the thrust
generated by the nozzle is proportional to the
throat area A t , reservoir pressure/^, pressure
thrust; and is a function of the pressure ratio
Po IPe ' U1( * the specific heat ratio 7.
Uith the definition of the thrust coefficient
C F by
Cp=F/(A t p 0 ), dimensionless (6-20)
Eq. 6-19 leads to the expression
where A e fA t is the nozzle expansion ratio. By
use of experimentally measured values of the
reservoir pressure p 0 , throat diameter, and
thrust F, the thrust coefficient is determined
from Eq. 6-20. This coefficient represents the
amplification of the thrust generated by the
expansion of the gas in the nozzle as
compared with the thrust that would be
generated were the reservoir pressure to act
over the throat area only.
If p e is less than p e , the pressure thrust is
negative. Nozzles are usually so designed as to
have the exit pressure p e equal to or slightly
higher than the ambient pressure p a In the
case of p e ** p e , Cp is known as the optimum
thrust coefficient and the nozzle is called a
perfect nozzle. It should be recalled, however,
that this ideal case can be valid only in the
event of irrotational, steady, isentropic flow
with 110 viscosity and no heat conduction.
Fig. 6-3 shows the variation of the
optimum thrust coefficient with the pressure
6-14
AMCP7M-238
Figure 6-4. Calculated Optimum Thrust Coefficient C F as a Function of Nozzle
Expansion Ratio e (y m 1.3)
ratio p 0 tPt for 7 ■ 1.2 and 1.3, while Figs.
64 and 6-5 show C F as a function of A e /A t * e
for 7 * 1.3 and 7 “ 1.2, respectively. In all
of these curves, C F refers to theoretical values
not incorporating any losses.
Because of the finite size of the chamber
area of cross section A e , a loss occurs in the
amount of the thrust generated by the nozzle.
In the event of an infinitely large reservoir, the
situation corresponds to a theoretical case of
ideal thrust generation since the thrust is
proportional to the product of the exit
velocity and mass flow of the gases. The losses
in thrust are progressively larger for succes¬
sively smaller ratios of chamber to throat
cross sections and for successively lower
pressure ratios p 0 /p t . For example, a straight
tubular chamber has approximately 22
percent less thrust than the equivalent ideal
chamber with infinite cross section. For a
chamber with an area ratio of 2 , the loss in
thrust is only 6 percent. These figures are for
a chamber pressure to nozzle exit pressure
ratio of 10 , and the influence of variation of
the specific heat ratio on the loss in thrust is
considered negligible.
6-7 DESIGN CONSIDERATIONS
In designing nozzles, various losses and the
associated correction factors are to be taken
into account for adapting the “one-dimen¬
sional” or hydraulic theory (O. Reynolds —
6-15
mmo a" ciTrrifVT w t , 7«ra^;^ sy:
AMCP 706-238
Figure 6-5. Calculated Optimum Thrust Coefficient C F as a Function of Nozzle
Expansion Ratio e fy= 1.2)
1885) of compressible flow to real nozzles.
These losses are due to the effects of such
factors as nonaxial and nonuniform flow,
imperfect gases, heat transfer, and friction.
In the convergent action of the nozzle, the
kinetic energy of the gases is relatively small
so that losses are very low, almost indepen¬
dent of the convergent nozzle shape provided
that the wall contour is symmetrical and
well-rounded particularly at the entrance and
throat sections. This is because discontinuities
give rise to shock waves in the nozzle.
In the divergent section of the nozzle,
however, nozzle losses depend markedly on
the configuration, shape, and angle of
divergence for a given expansion ratio e. Since
the rounding of the edge of the exit section of
the nozzle would lead to over expansion and
flow separation, this section customarily is
made to have a sharp edge. With increased
divergence at the exit, the radial component
of the velocity also is increased to result in a
reduction of the thrust generated by the
nozzle. Furthermore, in the event of a large
divergence at any single cross section of the
nozzle, losses due to separation, turbulence,
and divergence may become excessively high.
On the other hand, shor' nozzles with large
divergence give rise to low friction losses.
Contoured nozzles designed to give parallel,
uniform exit flow usually are excessively long
6-16
AMCP 706-238
and heavy and, in most instances, conical
shapes of divergence have been found
satisfactory because they are simple and
relatively easy to manufacture. The potential
flow in such a nozzle may be represented
closely by the flow due to a point source of
suitable strength and located at the vertex of
the cone. If 2a is the angle of the cone, a is
called the divergence angle of the nozzle.
Since thrust is based on the axial component
of momentum acting against the exit area
normal to the nozzle axis, the thrust of a
conical nozzle will be less than that of a
contoured nozzle. Simple geometric consider¬
ations give the ratio of the momentum flux in
the conical nozzle to that in the contoured
nozzle as
X = (1 + cos a)/2, dimensionless
( 6 - 22 )
The factor X is called the correction factor for
divergence angle a or, simply, the divergence
factor. The validity of Eq. 6-22 has been
confirmed by experiment for values of a up
to about 25 deg (Ref. 3). For a conical
nozzle, then, the thrust coefficient of Eq.
6-21 becomes modified as
(Cp^conictl
Another method of producing axially
aligned flow is by means of a plug nozzle.
Dependent on the particular plug configura¬
tion the expansion may be completely or
partially internal expansion (converging cen¬
tral plug); the performance is similar to that
of the conventional convergent-divergent
nozzle. In this configuration the nozzle length
is decreased by use of the plug, without
increasing the flow inclination; however,
because of the extra area, the skin friction is
increased. For complete internal expansion by
means of a conical plug (of cone angle 20) and
conical nozzle section, the flow may be
assumed to be emanating from a source
situated at the hypothetical intersection of
the plug and nozzle surfaces. With this
approximation and the additional assumption
that the vertex of the central conical plug lies
in the nozzle exit plane, the exit momentum
of such a flow can be evaluated by integrating
across the exit plane to give the divergence
correction factor as
_ (ain a + sin 0) 2 _
~ 2[(a + 0) ain 0 + cos 0 — cos a ]
dimensionless
(6-24)
where
a - angle of inclination of diverging nozzle
wall to axial direction (nozzle diver¬
gence angle), deg
0 = angle of inclination of converging plug
wall to axial direction, deg
It is seen that when 0 = 0, Eq. 6-24 reduces to
Eq. 6-22. When the outer wall is parallel to
the nozzle axis, a = 0 and Eq. 6-24 reduces to
, _ _ am P _
**** 2(0 sin 0 + cos 0-1)
(6-25)
It may be seen from these expressions that
conical plugs of large vertex angles can be
used without undue loss from flow diver¬
gence. In addition, the volume (and weight)
of the nozzle required to generate comparable
thrust can be less for a plug nozzle than for a
conventional nozzle, and cooling problems
may also be simplified. Plug nozzles of types
designed to create completely or partially
internal expansion have the added advantage
that the throat area can easily be varied to
compensate for changes in operating condi¬
tions (Ref. 4). Further information on the
performance of plug nozzles may be found,
for example, in Ref. 5.
Although many nozzles use a divergence
6-17
AMCP 706-238
angle of about IS deg, experience has shown
that the divergence angle a can be as great as
40 deg without incurring flow separation.
Consequently, the conical nozzle may be
reasonably short, quickly designed, and easily
produced. However, when a is 40 deg, Eqs.
6-22 and 6-24 show that a loss in thrust of
about 12 percent is incurred and, in many
cases, the advantage of short length is more
than offset by the thrust loss. A nozzle
accurately designed to give parallel flow at the
exit is theoretically :nore efficient than a
conical nozzle of the same expansion ratio
operating at the same exit pressure. The
design of such a nozzle (contoured nozzle)
involves an expansion section, in which the
flow deviates from the axial direction, as well
as a straightening section in which the flow is
redirected along the axis. In order to provide
shock-free flow axially aligned at the exit, the
nozzle contour must be designed accurately
by means of the method of characteristics.
Details of this method for axially-symmetric
flow are given in Refs. 6 through 9. Ref. 10
gives an approximate method for determining
the contour of the optimum-thrust nozzle by
purely geometric means. Nozzle contour
design and performance are also covered in
Refs. 11 and 12.
One of the disadvantages of the contoured
nozzle is that thrust loss is experienced when
the nozzle operates at pressure ratios and gas
compositions other than those for which it is
designed. In the event of an underexpanding
nozzle, exit pressure is greater than the
ambient pressure. In the case of an overex¬
panding nozzle, the exit pressure is smaller
than the ambient pressure. The effect of
either overexpansion or underexpansion is a
reduction in the exhaust velocity and a
corresponding loss of energy and thrust. In
highly overexpanded nozzles flow separation
occurs with the result that a large and,
usually, heavy portion of the nozzle is not
used; thus the nozzle is longer and bulkier
than required. In the absence of separation,
Eq. 6-21 is in good agreement with measured
results for underexpanded and for slightly
overexpanded nozzles. With a plug nozzle
where the plug is used so that all or most of
the expansion is external, the ambient air
behaves as the outer wall and thus gives this
type of nozzle the advantage of an automatic
adjustment of the exhaust expansion to
ambient pressure ar.d prevents loss due to
over- or underexpansion.
As for the convergent section of the nozzle,
losses are usually very low. In the event of
relatively steep convergent sections and
comparatively small radius-of-curvature
throats, however, losses are no longer
negligible, and the influence of the entrance
contour on the performance of such nozzles is
discussed in Ref. 13.
Ihe overall efficiency of a nozzle is based
upon the thrust it can produce and usually is
defined as the ratio of actual thrust to ideal
thrust, and is denoted by q. The ideal thrust is
based on the assumption of the one-dimen¬
sional flow of an ideal gas. A measure of how
closely an actual flow approaches the
condition of an adiabatic expansion of a
perfect gas is given by the kinetic energy
efficiency Vx t' defined as
v\
Vkb = 2(a35T > dimensionless (6-26)
where
v e = nozzle exit velocity (average value
over the cross section), fps
AH s theoretical specific enthalpy change
of gas from chamber to nozzle exit,
(ft-Ibi-slug" 1
The values of efficiency have the range
0.90 < t) KE < 0.99. The efficiency of a
nozzle may be affected by the nonuniform
distribution of both the magnitude and direc¬
tion of the velocity across the exit, and an
associated velocity coefficient % is defined as
q„ = 7— -, dimensionless ( 6 - 27 )
UVtJkMrvtfcal
4
i
*
618
AMCP 706-238
where the effective velocity (v e ) effective
could properly replace (v e ) lheoretiail in the
one-dimensional equations. The discharge
correction factor n d is defined as the ratio of
the actual mass flow rate G' a in a real nozzle
to the ideal critical flow rate G' at the throat
of the nozzle working with the same inlet
conditions and the same exit pressure, i.e.,
Vd - G'jG' t dimensionless (6-28)
From Eqs. 6-28 and 6-17 it follows that
G' a \'yKTJ
dimensionless (6-29)
The value of r\ d varies from 0.98 to 1.15. The
reason that t) d is usually greater than one is
based on the following facts:
1. The molecular weight of the gases
increases slightly when flowing through a
nozzle, thereby changing the density.
2. The heat transferred to the walls lowers
the gas temperature and raises the density.
3. The change in 7 down the nozzle is such
as to increase r\ d .
In summary, for a nozzle operating with
P e = P a > since the thrust is given directly by
the momentum of the exhaust gases, the
overall nozzle efficiency r, may be expressed
by
V = (Vics) 1/2 (Vv>(Vd^ (6-30)
In general, the value of ij can vary from 0.85
to 1 . 10 , depending on design parameters.
6-19
AMCP 706-238
SECTION III
THEORY OF RECOIL CANCELLATION
6-8 DEFINITION OF MOMENTUM RATIO
PARAMETER
Fr = A b p c \l
The analysis of flow of propellant gas with
entrained solid propellant grains is very
complex and beyond the scope of the present
handbook. In all of the standardized recoilless
rifles in the US, the perforated cartridge case
helps to confine the propellant grains so that
most of the charge is consumed in the
chamber (or rifle). In the 57 mm Ml8
Recoilless Rifle, the solid propellant ejection
through the nozzle was experimentally
determined to be approximately 0.3 lb (or
about 30% of the total charge). The amount
of unburnt propellant does not vary appreci¬
ably from round to round at a given
temperature so the performance of the rifle is
not affected. It should be pointed out that
from the logistic standpoint, elimination of
solid propellant ejection would lead to a
smaller and lighter round. However, even in
the recoilless rifle with nozzles in front of the
chamber and initially closed by the projectile
(Front Orifice Recoilless Rifles such as 105
mm T135), approximately 10 percent of the
propellant charge is expelled unburnt. In the
Battalion Antitank Weapon (BAT), the M40
Recoilless Rifle unburnt propellant ejection is
about 20 percent.
From a design standpoint, the nozzle of a
recoilless rifle can be designed in accordance
with this chapter. The unburnt propellant
ejection does not affect appreciably the
dimensions of the nozzle designed on the
basis of gas flow only.
/ 2 7 2 / 2 y^D/'r-D
7M (r ’ 1>/r T
Jy-i\y+l)
\Po) J
lb
(6-31)
where
A b = bore area of rifle, ft 2
A e = exit area of nozzle, ft 2
A ( = throat area of nozzle, ft 2
F r = force of recoil, lb
p Q = ambient pressure external to rifle,
lb-fr 2
p c = chamber pressure, lb-ff 2
p e = pressure at exit section of nozzle,
lb-ff 2
p 0 = pressure in a large reservoir from
which the flow in nozzle could have
arisen by purely isentropic flow,
lb-ff 2
7 ~ c p! c v ra ti° °f specific heats, dimen¬
sionless
Denoting by F R the instantaneous total
force on the rifle, which produces the recoil,
and assigning to it the positive sign for
rearward recoil, Ref. 14 derives the following
expression for F R :
The last term ( A e - A b ) p a > in Eu. 6-31,
which is the force contribution due to
ambient pressure, is generally small and
usually can be neglected. This expression is to
be considered approximate in the sense that
0-21
Preceding page blank
AMCP 706-238
its derivation is based on certain simplifying
assumptions. For practical purposes, however,
it is of sufficient accuracy consistent with
those of the fore 0 >oing. It is noted that Eq.
6-31 can be recovered more directly as
~ F (6-32)
where F is the instantaneous thrust generated
by the nozzle and given by Eq. 6-19. It is
apparent from Eq. 6-32, that one of the
simplifying assumptions employed in the
derivation of Eq. 6-31 is that of the projectile
base pressure being equal to the chamber
pressure.
In order to estimate the total impulse
delivered to the rifle, Eq. 6-31 is integrated
from t - 0, when it is assumed that the
projectile is free and the nozzle open, to time
t m when the projectile leaves the muzzle, ""he
dimensionless recoil (momentum ratio param¬
eter) w is then introduced by the definition
co =
r*m
J, F »*
J 0
(6-33)
By referring to this definition, it is seen that
the numerator is the impulse delivered to the
rifle while the projectile is in the barrel (i.e.,
rifle momentum of recoil). For a large class of
recoilless rifles, i.e., those in which the
pressure on the projectile base is nearly equal
to the pressure of the chamber, the
denominator of Eq. 6-33 is nearly equal to
projectile momentum at the muzzle. Thus co,
the dimensionless recoil, is for a large group
of rifles equal to the ratio of rifle momentum
(positive to the rear) to projectile momentum
at the instant of projectile ejection; hence, the
t^rm “momentum ratio parameter”.
6-9 EQUATION FOR MOMENTUM RATIO
AS A FUNCTION OF GUN AND
NOZZLE PARAMETERS
Neglecting the term (A e - A b ) p g . in Eq.
6-31 and combining it with Eq. 6-33, and
using Eq. 6-12, one obtains
A Pc / 1 \)/ 2 yr*i)/C2<r-i) j
A ' p 0 ~Vi-Wi\Y + V
(6-34)
where the ratio p e lp 0 must satisfy Eq. 6-15.
For each value of p e /p 0 , a value of the nozzle
expansion ratio AJA, is obtained from Eq.
6 -12, and a value of the ratio
( A h /A,) • (p c /p 0 ) is obtained from Eq. 6-34 for
a given value of co. Thus, the dependent vari¬
able of Eq. 6-34 is a function of the expansion
ratio e of the nozzle, the quantities co and y
being parameters. The curve represented by
this function is called a line of constant dimen¬
sionless recoil. For y ~ 1.25 and for values of co
from - 0.10 to 01 Oat increments of 0.01, Fig.
6-7(A) gives the lines of constant dimensionless
recoil.
6-10 EQUATIONS FOR RATIO OF CHAM
BER PRESSURE TO IDEAL RESER¬
VOIR PRESSURE
The momentum equation in steady flow
6-22
WMwawgaiipp
i
)
AMCP 706-238
between a postulated plane of zero gas
velocity in the cylindrical chamber (see Fig.
6-9) and the plane of intersection of the
nozzle with the chamoer-inlet section of the
nozzle-is
G'Vi = ■A e (p c — p f ) (6-35)
where the subscript i refers to quantities at
the inlet of the nozzle, and the mass flow rate
G' is given by Eq. 6-17. Substituting into Eq.
6-35 the value of v, from Eq. 6-4 in which
p = Pj, one has
& I ** ( l V rn>Ay ~T. M "" h T
P,~Po A,J(Y-l)[v *l) L U/ J
(6-36)
As in the case of Eq. 6-11, use of Eq. 6-5
leads to
and the substitution of this into Eq. 6-36
results in
5- '/WKT-feU—
where
Pi ( 2 y/(r-l)
p 0 \y + 1 /
If Pilp 0 is eliminated between Eqs. 6-37 and
6- 38, p e /p 0 is expressed as a function of
A e /A t . The curve representing this function is
shown in Fig. 6-6 for 7 = 1.25.
6-11 GRAPHICAL SOLUTION OF THE
EQUATIONS
In the problem of design where the throat
area A t is sought, the relation between bore
cross-sectional area A b and chamber cross-sec-
tiona! area A c usually is known. Let A c = r'A b ,
where the dimensionless constant r ( - AjA b
is known (typical values of r t may vary
between 2.0 and 3.0). Fig. 6-6 then gives the
relation between p c lp 0 and r' (A b /A t ), and
one can obtain the corresponding values of
Pc/Po and A b /A, - r' A c /A r By use of this
relationship, the factor p c /p 0 appearing in the
ordinate of Fig. 6-7 (A) can be eliminated to
obtain the corresponding A b /A, versus A e /A t
curves of constant dimensionless recoil co, all
for a particular value of r'. This set of curves
is presented in Fig. 6-7 (B) for the special case
of r' » 1 (bore cross section being equal to
chamber cross section). It is interesting to
note that for zero recoil (o> = 0 ) and with
A f - A t , Fig. 6-7(B) gives the required ratio of
A b /A t = 1. This is the straight tube, open at
both ends, which often is cited as an example
of a recoilless rifle.
If at any point on a curve of Fig. 6-7(B) the
I abscissa is divided by the ordinate, one obtains
the ratio A e /A b . The relation A e fA b - constant
appears as a straight line in Fig.
6-7(B) where these are shown in dashed lines
ioxA e lA b - 1.5,2.0,2.5, and 3.0. Along such
a line, the throat cross-sectional area varies
while A b and A e each can be considered
constant. Nozzle erosion results in an increase
in A f with little, if any, change in A b and A e .
Obviously, then, the change in recoil which
accompanies nozzle erosion may be predicted
by moving down the line of constant A e /A b
starting at the initial (uneroded) values of A,
and co for the rifle. The A e /A b = constant
type of a straight line is also useful in
estimating the change in throat area necessary
to eliminate excessive recoil.
6-23
mmauawmiw
AMO 706-238
F/gune 6-6. Chamber Pressure/Ideal Reservoir Pressure as a Function of Chamber
Area/Nozzle Throat Area («t= 1.25)
To sum up, by use of Fig. 6-6 and Fig.
6-7(A), a corresponding set of curves (as in
Fig. 6-7(B» is constructed for a given value of
r = A c /A b and for a selected range of values
of cd. The resulting figure, together with the
appropriate lines of A e /A b = constant, is
valuable in actual problems of recoil consider¬
ations of guns.
6-12 NOZZLE PERFORMANCE FACTORS
6-1Z1 VARIATION OF NOZZLE THRUST
WITH NOZZLE EXPANSION AN¬
GLE
An important consideration in the design
of a recoilless rifle is the nozzle expansion
angle 2a which is twice the divergence angle
treated in par. 6-7. In rear orifice recoiliess
rifles, a larger expansion angle means a shorter
nozzle for a given expansion ratio and, hence,
a corresponding saving in nozzle weight. In
some front orifice recoilless rifles (see Ref.
26), a long nozzle is required to carry the
gases beyond the rifle breech. Hence, a small
expansion angle is desirable to obtain a small
frontal area and minimum weight.
The effect of the angle a on nozzle thrust
F , which is given by Eq. 6-19, is represented
by the divergence correction factor X of Eq.
6-22. As expressed by Eq. 6-23, nozzle thrust
varies linearly with X. For a perfect nozzle
( P e - p a ), F becomes proportional to X for a
given expansion ratio. The percentage loss in
thrust then becomes 100(1 - X). For a
conical nozzle. Table 6-2 shows the variation
of this loss with the angle 2a.
It should be noted that, since Table 6-2 is
6-24
AMCP 706-238
(A) (Bore Area/Nozzle Throat Area) x (Chamber Pressure/Ideal Reservoir
Pressure) as a Function of Nozzle Expansion Ratio (7 = 1.25)
Figure 6-7. Lines of Constant Dimensionless Recoil w
based on Eq. 6-22, the validity of which has
been confirmed by experiment (Ref. 3) for
values of a up to about 25 deg, this table is
valid for 0 < 2a < 50 deg. It thus is
concluded that the decrease in nozzle thrust
due to expansion angles not exceeding 50 deg
is less than 5 percent.
As to the influence of the expansion angle
on unbalanced forward recoil force - F R ,
given by Eq. 6-32, results of tests (Ref. 15, p.
63) conducted in the differential thrust bomb
on nozzles with expansion angles of 5, 15, 30,
45, and 60 deg, and expansion ratios of e - 4.5
indicate that angles less than 45 deg give
essentially the same (- F k I(Ai ,[) c )I (100)
value but, in the event of 2a > 45 deg, there
is a significant decrease.
6-12.2 VARIATION OF NOZZLE THRUST
WITH EXPANSION RATIO
The results of a study (Ref. 15, p. 65) of
the effect of nozzle expansion ratio on the
recoil force of nozzles are summarized in
Table 6-3 where both experimental and
theoretical values are given for the variation
of percent rearward recoil force imbalance
l F R /(A b p c )] (100), with nozzle expansion
ratio. Various nozzles having circular throat
AMCP 706-238
(B) (Bore Area/Nozzle Throat Area) as a Function of Nozzle Expansion
Ratio (7 = 1*25)
Figure &7. Lines of Constant Dimensionless Recoil cj
sections and radial expansion cones with
expansion ratios of 1.0, 3.IS, 6.77, and 9.68,
and identical throat areas were tested in the
differential thrust bomb. Except for the case
of € * 9.68, the experimental and theoretical
data (based on a discharge coefficient of 0.94)
of Table 6-3 are in good agreement. The
discrepancy in the event of e = 9.68 is, in
part, due to flow separation, and increased
friction and heat losses.
In a recoilless rifle, as previously noted, the
increase of the forward recoil force imbal¬
ance, which results from the erosion of the
nozzle throat, shortens the useful life-span of
its nozzle. The nozzle life is determined by
the acceptable level of augmentation in recoil
force imbalance, and can be predicted
provided that the erosive properties of the
nozzle material and the change in recoil force
imbalance with change in throat area are
known. Fig. 6-8 shows the close correlation
between the experimental and theoretical
results (based on a nozzle discharge coeffi-
6-26
AMCP 70S-238
TABLE 6-2
VARIATION OF NOZZLE THRUST WITH
NOZZLE EXPANSION ANGLE 2 a
Nook Expansion
Loss In
An&t Tot, dag
NozzJs Thrust, %
0
0.00
10
0.20
20
0.76
30
1.70
40
3.02
50
4.68
60
6.70
70
9.04
80
11.70
90
14.64
cicnt of 0.94) on the variation of recoil force
imbalance with nozzle throat area (Ref. 15, p.
68 ).
6-12.3 EFFECT OF NOZZLE APPROACH
AREA AND CHAMBER CONFIG¬
URATION ON RIFLE PERFOR¬
MANCE
In order to study the effect of chamber
configuration on recoilless rifle operation, a
series of firing tests was conducted on the 57
mm Recoilless Rifle, M18 (see Ref. 15),
employing various internal chamber config¬
urations and nozzle entrance areas of cross
section. The M18 chamber was modified by
use of ? variety of five liner inserts described
in Ref. 15. For these tests the chamber
volume was kept constant at 80 in?, and a
charge of M2 Propellant, Lot RAD 459, was
contained in a standard S7 mm perforated
cartridge case. The nozzle used had a throat
area A, of approximately 3.0 in? with an
expansion ratio of e = 2. The ballistic data
obtained for the rifle fired with the liners are
given in Table 6-4. This table indicates that, as
the nozzle approach area A, is decreased, the
rearward recoil imbalance increases. A plot of
TABLE 6-3
VARIATION OF RECOIL FORCE IMBALANCE
WITH NOZZLE EXPANSION RATIO
Percent Rearward Imbalance of
_ Recoil Force
Experimental
Expansion
Ratio €
(avarags ot four
rounds)
Thaoratical
1.00
+ 4.0
+ 4.0
3.15
-18.6
-18.6
6.77
-26.5
-27.0
9.66
-29.2
-30.6
this variation versus the ratio of nozzle
approach area to nozzle throat area is shown
in Fig. 6 -9. It is seen from this plot that, for a
given nozzle throat area, percent recoil
imbalance is an inverse, nonlinear, function of
the nozzle approach area, the imbalance
increasing at a progressive rate with decreasing
approach area.
The experimental plot in Fig. 6-9 is
correlated by a theoretical plot also shown in
the figure and obtained from Fig. 6-6. It is
noted that the two plots have a
close similarity through the range of
Aj/A, - A c /A r the divergence of the curves
occurring at area ratios lower than 2. This
divergence is explained by the difference of the
“effective” nozzle approach area, attributed to
chamber configuration and heat losses.
It is seen from Table 6-4, that chamber
configurations such as employed with cham¬
ber liners Nos. 2 and 3 do not give
satisfactory recoil compensation, and it is
concluded from Fig. 6-9 that the ratio o f
nozzle approach area to throat area should be
equal to, or greater chan, 2.0. For recoilless
rifles, the bore area to throat area ratio is, in
general, approximately equal to 4/3. This
would give an approach-area to bore-area ratio
of aporoximatcly 1.5, or greater, as used, for
6-27
AMCP 706*238
Figure o-ft Percent Recoii Force Imbalance as a Function of Nozzle Throat Area
example, in tie design of the 75 mm considerably higher with liner No. 5 than with
Recoilless Rifle, T41. liner No. 1, which has a fully tapered
configuration, even though they both have
Due to the close radial confinement of the the same geometric approach areas. The
burning charge in the front portion of the maximum pressure is approximately 30%
chamber, the chamber design given by liner greater and the recoil imbalance is 2.5%
No. 5 results in chamber pressures liiglier than greater for the configuration of liner No. 5.
for an equal volume chamber with a uniform
annular space as exemplified by chamber liner Results of similar studies repeated with the
No. 3. Liner No. 5 also gives the highest 57 mm Recoilless Rifle, Ml8, fitted with a
ballistic efficiency (i.e., ratio of projectile centrally-located circular nozzle (in lieu of the
idnetic energy to total propellant energy) annularly-located kidney nozzle) indicate that
when compared with liner Nos. 1 and 4. (see Ref. 15):
Furthermore, it is to be noted that the
maximum pressure and recoil imbalance are 1. The expansion of gases in tne central
6-28
AMCP 70&238
O
a
\
Pu
a>
3
m
u:
<D
<D
03
U
•—4
<d
a>
5
0 )
t_
G
03
03
0 )
<D
XI
s
(d
x:
U
1.40
1.30
h 6
1.20
1. 10
1.00
c
0 )
2
03
04
( 1 )
o
c:
<d
o
o
<D
Cd
r v
(6
(6
0 )
a:
3
(d
Q
^-4
(d
4_>
G
, <D
f I
t-.
0 )
r
>,
w
Nozzle Approach Area/Nozzle Throat Area A i /A t or
Chamber (Uniform) Area/Nozzle Throat Area A_/A f
C l
Figure 6-9. Effect of Approach Area A. on Recoil In,balance of the 57 mm
Recoi.'less Rifle, M18
nozzle is slightly more efficient than in '.he
kidney-shaped nozzle.
2. The recoil compensation is less sensitive
to change in geometric approach area for the
central nozzle as compared to the kidney¬
shaped nozzle in the critical region between
ratios of approach area to throat area of one
to two.
3. The ballistic efficiency of the rifle is
slightly greater for the central nozzle than for
the kidney-shaped nozzle.
AMCP 706-238
TABLE 6-4
BALLISTIC DATA FOR THE 57 mm RECOILLESS RIFLE, M18, FIRED WITH
VARIOUS CHAMBER CONFIGURATIONS (Rtf. 15)
Rearward Nozzle
Irutrim ant Maximum Recoil Approach Propellant Ballistic
Linar Charge, Velocity, Pressure, Imbalance, Area, A f Loss, Efficiency
No*
0
fps
P«
%
in?
%
%
1
426
1232
7200
4.5
9.0
25.2
3.46
aoo
2
360
1282
7450
36.0
3.05
20.0
4.43
1.02
3
405
1282
7900
17.7
4.37
23.0
3.94
1.46
3
426
1366
9100
17.4
4.37
21.8
4.22
1.46
4
426
1272
8050
8.6
6.0
23.6
3.66
2.00
5
426
1294
94Cw
7.0
9.0
24.2
3.82
3.00
A t - Nozzle throat area - 3.0 in? Propellant: M2, Lot RAD 450
Chamber volume - 80 in? Cartridge Case: 57 mm, M30
•Liner No. 1: similar to conventional tapered chamber rifles with a nozzle approach area 3.0 times throat
area
Liner No. 2: ravened taper and a nozzle approach area of 1.02 times throat area
Liner No. 3: constant cross section, with a nozzle HV.oach area 1.46 times throat aria
Liner No. 4: tapered, with an approach area 2.0 times throat area
Liner No. 5: constant cross section, but only on>half the chamber length. The nozzle approach area
equals that of the 67 mm, Ml 8 chamber, or 3.0 times the throat area
6-30
AM CP 706*238
SECTION IV
NOZZLE EROSION
6-13 GENERAL DISCUSSION
Of all phenomena unfavorable to long
nozzle life, nozzle erosion is the worst
offender. Erosion rs the progressive wearing
away of the inner surface of the nozzle as the
gun is used. It is greatest at the throat section
and, as the bore of the throat tends to
become enlarged, the effect is to diminish the
rearward recoil to zero and even ultimately
generate forward recoil.
The erosion process is very complex and its
details are not fully unde r stood. The process
involves mechanical, chemical, and thermal
effects which are interrelated. Erosion is
primarily a physical activity although chemi¬
cal action can increase its rate. The abrasive
effects of propellant gases and particles
impinging at high velocities on the nozzle
surface are highly damaging because they
sweep away some of the nozzle surface
material. This phenomenon is known as gas
wash. Erosion is particularly sensitive to the
heating of metal surfaces. By melting a very
thin layer of the nozzle surface, and thereby
making it easier for the gases to carry off the
nozzle surface material, intense heat con¬
tributes indirectly to the process of erosion.
At high temperatures, some constituents of
the propellant gases may undergo chemical
combination with the nozzh surface material
to form brittle compoui ’ that may crack
and peel off under the action of propellant
gases.
Erosion is particularly severe if hot-burning
propellants are used. Under these circum¬
stances, the erosion induced by the gases is
most significant. As the flame temperature is
increased, the erosion rate increases much
more rapidly than the rate of increase of the
flame temperature to such an extent that the
thermal effects become dominant. Another
form of damage to the nozzle as a result of
firing is that of the large thermal and
mechanical stresses developed. The result is
that the nozzle surface develops a characteris¬
tic pattern of cracks which lead to a
developing roughness tliat increases the heat
transfer to the nozzle. These cracks erode
locally, and the surface ultimately becomes
quite rough. In contrast, low-energy weapons
using cool propellants erode very slowly.
6-14 THEORY
Erosion is an ever-present problem in
recoilless guns. The phenomenon of erosion
has been studied quite extensively, but only a
portion of the literature is useful (Ref. 16).
Many articles written on the subject of
erosion in guns elaborate the theories that
have been advanced, although there is little
reliable experimental data available.
One method of study of the erosion
process has been to allow the hot propellant
gases to flow through a hole or vent in a block
of known weight and determine the weight
loss. This method is applicable to weapons
using nozzles, and the results show that
erosion in vents is of two types: (1) a melting
type found above a minimum density of
loading, and (2) a chemical type (bund below
this minimum. In the chemical type of
erosion, chemical composition of the ingredi¬
ents, particularly of the primer, is found to be
of importance. A comprehensive study
concludes that erosion is due to the melting
of the surface and shows that this melting
starts at a definite pressure independent of
the size of the chamber in which the
propellant is burned. Above this pressure, the
rate of increase of erosion per unit of added
charge is independent of chamber size.
However, correlation between weight of
eroded material and charge weight appears to
be better.
6-31
AMCP 706-238
The vent plug test-either as a cylindrical
plug with a hole, or as two flat plates of
different material with the gas passage
between, or as several rings of metal forming
one tube—appears most analogous to erosion
in nozzles. Increasing the pressure in the
vented bomb at first causes but slight erosion
up to a critical pressure, while further
increase causes a rapid increase of erosion in a
linear manner. Increasing the area of the hole
causes a decrease in erosion, approximately
proportional to the area. This is an indication
of the effect of the length of time of contact.
Increasing the flame temperature of the
propellant increases the erosion in a linear
manner, roughly ? 10 percent increase in
erosion per 100 deg C increase in flame
temperature.
The erosion of metal by the hot gases
produces a characteristic appearance of the
metal surface. In ferrous metals, the surface
takes on a checked appearance, while in
nonferrous metals the surface has a streaked
washed-out appearance. Metallographic exam¬
ination of ferrous alloys shows several layers,
beginning with one or more white layers at
the surface, of martensitic structure, with
lower layers of troostitic and sorbitic
structure.
Nonferrous alloys show some changes in
grain structure and, of course, mere of the
martensitic appearance. While the checked
structure usually accompanies erosion, it is
not certain whether or not if is a cause of
erosion. One theory is that the checked
structure results from the rapid cooling of the
metal surface following the heating by the hot
gases.
The two main theories of erosion, then, are
the surface fusion theory, and the chemical
reaction theory. Supporting the surface fusion
theory is the relation among the melting point
of the metal and the amount of erosion, and
the agreement with heat transfer theories.
Chemical reaction theory may apply in vent
plugs only below some critical pressure at
which melting starts.
6-15 EROSION RESISTANCE O* VARI¬
OUS METALS
In the design of the nozzle of a new
recoilless rifle, the designer must be supplied
with reliable design data with respect to
material, configuration, and dimensions of the
nozzle for a projected performance of the
gun. In line with this objective, the results of
a comprehens've program of study on nozzle
erosion are summarized in Ref. 17. Included
in this study is a theoretical analysis of the
transfer of heat to nozzles, which leads to a
useful classification of engineering metals and
metallic alloys on the basis of the surface
melting under conditions of gun firing,
followed by an extensive experimental pro¬
gram to evaluate the erosion of nozzles
manufactured from all of the known promis¬
ing materials in order to correlate the
experimental data with the thermal properties
of the materials.
The nozzle erosion tests described in Ref.
17 were carried out in a vented bomb,
constructed from a 37-mm breech with a
chamber volume of 19.5 in?, with a standard
shape nozzle. The rate of increase of nozzle
throat, rather than the weight loss per round,
was adopted as a criterion of erosion. By use
of the Ml, M10, and M2 Propellants-the
isochoric flame temperatures of which are
2580% 3040% and 3510% respectively-
the experimental results pertinent to erosion
resistance of various materials are summarized
as follows:
1. Molybdenum:
Pure molybdenum possesses the highest
erosion resistance of all the materials tested.
Tliis is in agreement with predictions by the
theoretical classification of metals based on
their thermal properties.
Porosity in molybdenum increases the
relative erosion rate appreciably. Erosion of
pure molybdenum is characterized by deep
radial cracks and some spalling. Localized
melting along the cracks is apparent.
6-32
wmmnmm
AM CP 706-238
(
i
i
i
2. Sintercast:
Nozzles of composition of 60% Mo and
40% Cu and with a throat lining of either
molybdenum or chromium behave in much
the same way as pure copper does, indicating
that-after the throat line is blown off -ero¬
sion is probably caused by the melting of the
copper and subsequent spalling of the
molybdenum particles. The nozzles have a
roughened appearance characteristic of this
type of erosion.
Nozzles of composition of 80% Mo and
20% Cu and with a throat liner of cither
molybdenum or chromium exhibit approxi¬
mately the same relative erosion rates under
continued firing conditions. Before the
chromium liner completely erodes away after
IS rounds at 35,000 psi, the chromium-lined
throat walls are somewhat superior to those
with molybdenum liners.
3. Tantalum:
The erosion of tantalum is found to be
higher than expected on the basis of the heat
transfer theory alone. The surface of the
tantalum liner becomes darkened under firing,
pointing to a change of chemical nature.
Unlike molybdenum, however, no signs of
spalling or cracking exist. Under conditions of
rapid fire at 35,000 psi, the rate of erosion of
tantalum is found to be more than twice that
of molybdenum.
4. Tungsten Carbide. Tungsten carbide
nozzles employing approximately 5% Co as a
binder exhibit appreciable erosion at the
entrance section and along the external edges
of the nozzle because of the melting of the
cobalt binder, while undergoing less than one
percent increase in the throat area at pressure
levels of up to 30,000 psi. At 35,000 psi.
however, complete shattering of one and
serious damaging of another nozzle has been
recorded.
5. Copper and Copper Alloys:
Pure electrolytic copper compares very well
with gun steel and possesses excellent erosion
characteristics at pressures below 20,000 psi.
After 40 rounds at this pressure and with the
M2 Propellant, no appreciable erosion is
observed, as compared with gun steel that
erodes 10% after 15 rounds at 20,000 psi.
Because of its relatively low mechanical
strength, copper is not recommended for use
at pressures in excess of 20,000 psi. While the
change in throat area is not excessive at
30,000 psi with the M10 Propellant, the
extrusion of the copper-lined nozzle and the
deformation of the solid copper nozzle are
appreciable. Below 20,000 psi, however, the
deformation of copper is less than that of gun
steel.
Alloys of copper erode somewhat more
rapidly than pure copper but, unlike copper,
do not erode appreciably at 30,000 psi. This
is to be expected because slight amounts of
alloying elements cause a reduction in the
thermal conductivity while increasing the
strength.
6 . Gun Steel. Undei tests with the M2
Propellant, gun steel (SAE-41S0) exhibits
negligible erosion at 10,000 psi. At 20,000 psi
and 30,000 psi, the increase in throat area is
3.4% and 8.4%, respectively, for five rounds
of firing. These figures are higher, approxi¬
mately by one order of magnitude, than the
corresponding ones for copper.
7. Cast Steel. A cast steel having a
composition comparable to SAE-4340 exhib¬
its the same order of magnitude of erosion as
that of gun steel, when subjected to the
standard erosion test with the M2 Propellant.
The actual figures arc somewhat lower in the
case of cast steel. Furthermore, this steel is
exceptional in that the amount of erosion
with the Mi Propellant (flame temperature
2580°K) is almost identical to that with the
AM CP 706-246
M10 Propellant (flame temperature 3010°K).
This is conflrmed by a theoretical analysis
(Ref. 17) based on heat transfer and surface
melting considerations, showing that a com¬
bination of conditions can exist under which
the erosion rates will be the same despite the
different flame temperatures.
8 . Stellite. This is a cobalt-base alloy used
for facing valves and high-speed cutting tools.
Stellite maintains a high tensile strength even
at red heat. All four alloys of stellite tried as
nozzle material suffered extreme erosion. The
thermal conductivity of these alloys is so low
that the surface tempeiature of the nozzle
probably reaches the melting point.
9. RAF Styria Stainless Steel. Results of
tests with the best of these stainless steels
indicate that this material does not have good
erosion characteristics. Nozzles made from a
sample of the best RAF styria stainless steel
can undergo an erosion of 26% when
subjected to the standard test procedure at a
pressure of 30,000 psi.
10. Titanium Carbide. Titanium carbide
nozzles also appear to have considerably less
erosion resistance than gun steel. A sample of
a cobalt-bonded titanium carbide with smaller
amounts of other ingredients to promote
oxidation resistance is reported to have
undergone an erosion of 11.5% after 5 rounds
at 10,000 psi and 5 rounds at 20,000 psi with
the M2 Propellant.
11. Timken Alloy. The principal alloying
elements in Timken alloy arc approximately
16% Cr, 25% Ni, and 6% Mo. On the basis of
weight loss, the erosion of Tirnken alloy
nozzles fired with the M2 Propellant at a
nominal pressure of 20,000 psi is reported to
be 0.42 g/round. The corresponding figure?
for gun steel, cast steel, and cast molybdenum
are 0.165, 0.08, and 0.002 g/round, respec¬
tively.
12. Graphite. It is reported in Ref. 17 that
a nozzle made from pure carbon-bonded
graphite shattered after one round of testing
at. 10,000 psi. Graphite does not possess
sufficient mechanical strength for this applica¬
tion.
13. Titanium. When a pure titanium-lined
nozzle is tested at 15,000 psi with M10
Propellant, it erodes very severely. The type
and degree of erosion indicate the occurrence
of a chemical reaction between the titanium
and some of the constituents of the
propellant gases.
14. Ceramics. The mechanism of erosion in
ceramic nozzles is quite complicated. Two
theories of the erosion of ceramics have been
advanced: spalling, and thermal shock. Spall¬
ing is caused, at least in part, by the
penetration of gases into small cavities and
their subsequent expansion after the pressure
is released. Consequently, porosity is an
important factor in these nozzles. In the
thermal shock theory of erosion, localized
failure of the material occurs because of the
sudden rise of the surface temperature of the
nozzle throat. The surface temperature of
ceramic nozzles will reach practically the gas
temperature in 2 to 5 msec. For adiabatic
flow with M2 Propellant, the gas temperature
is approximately 2300°C at the throat and
may even exceed 3000°C should combustion
occur there. Even in the best ceramics,
erosion is characterized by spalling and
apparent melting at the surface, and the best
of the ceramic nozzles are found to be
inferior to gun steel.
15. Coated Materials:
In the coating of nozzles, a thin liner or
inner surface coating, usually of some
material with a high melting point or
particularly desirable chemical resistance
properties, is backed by a material having
good thermal properties. Experimental studies
of the possibility of using composite nozzles
to reduce nozzle erosion have been very
f'.nited. The results of tests on sintered
molybdenum nozzles with oxide-resistant
6-34
AMCP 706*238
coating indicate that it is doubifui that the
coating has any more than a temporary effect
in reducing erosion, since the increase in the
average radius at the throat is considerably
greater than the thickness of the coating.
Aluminum alloy and magnesium alloy nozzles
equipped with special coatings are reported
also to have undergone complete and severe
erosions, respectively (Ref. 17).
In order to estimate theoretically the
amount of erosion of the nozzle due to
surface melting, the following approximate
expression, derived in Ref. 17, for the
temperature of the surface is useful:
ar. 2(ht in )
AT, (irfcp'c) ^ +3 (A/^ 2 ) '
dimensionless (6-39)
where
AT S - temperature rise of inner surface of
nozzle throat (with reference to
nozzle initial temperature), °R
AT t = temperature rise of propellant gas at
nozzle throat (with reference to
nozzle initial temperature), °R
which is the case in gun vents and barrels, the
deviation of Eq. 6-39 from the exact solution
is reported to be less than 2 percent (Ref. 17).
If AT m = difference between melting
temperature of nozzle material and initial
temperature °R of nozzle, and %MP = percent
of melting point, then
2 (ht in )
(TTfcp'c) 1 ' 2 +f(Af l72 )
(6-40)
On the basis of Eq. 6-40, a theoretical
classification of metals is made for their use as
erosion-resistant materials. For heat transfer
coefficient values of h = 2, 4, and 6
cal - (cm 2 - sec - °CT l , and for an exposure
time to the hot gases cf 5 msec, Eq. 6-40 gener¬
ates the curves of Fig. 6-10. This figure in¬
dicates that the pure metals tungsten, moly¬
bdenum, tantalum, iridium, chromium, and
copper are the most promising metals since
they exhibit lower %MP values for the magni¬
tudes of h and / considered. This conclusion is
in agreement with experimental results of ero¬
sion studies.
h = heat transfer coefficient from pro¬
pellant gas to nozzle surface,
caHcm 2 -sec-°C)' 1
k = thermal conductivity of the nozzle
material,
caHcm 2 -scc-°C/cm) '*
p' = density of nozzle material, g-cnT 3
c -specific heat of nozzle material,
cal-fg^Cr 1
/ = time, sec
Eq. 6-39 has the advantage of showing the
surface temperature rise as being practically
proportional to the heat transfer coefficient
for small values of time. For t = 5 to 10 msec,
Since the surface temperature rise of these
nozzles is approximately inversely propor¬
tional to (kp'c) l/2 , ceramics to be used in
making the nozzles should have a large kp c
product. Although the product p c for
ceramics may be of the same order of
magnitude as for metals, for most ceramic
materials A is 10 to 100 times smaller than for
metals. For an advantageous application,
then, the constituents of the ceramic nozzles
must have high melting points preferably of
the order of 3000°K. This would minimize
the major cause of the severe erosion of
ceramic nozzles believed to be due to the
melting of the throat suiface where the
temperature rise reaches the level of the gas
temperature in a very short period of time.
In summary, nozzle materials fall into two
6-35
AMCP 706-233
O r
LEGEND
0
Zinc
r
Aluminum
ft
Stellite
%
a
80 Nl-20 Cr
$
18 Cr-8 N (re)
A
Monel
(66 Nl. 30, Cu)
V
SAE-4150
•
Paladium
ft
Iron
a
Nickel
A
Platinum
T
Silver
0
Chromium
V
Copper
ft
Iridium
X
Tantalum
□
Molybdenum
o
Tungsten
Figure 6-10. Theoretical Classification of Metals cn the Basis of Heat Transfer Properties
general classifications: the heat-absorbing
type and the heat-resisting type. To the
heat-absorbing class belong the simple metals
and alloys which, because of their higher
thermal conductivity, are able to remove the
heat from the surface of the nozzle. To the
heat-resisting class belong materials like
ceramics, having low thermal conductivity
and high melting points. Ceramics as a class
have been found to lack sufficient mechanical
strength to withstand the erosive forces of the
gases.
6-16 SIMILITUDE RELATIONSHIPS
For the purpose of extending the experi¬
mental erosion results obtained on nozzles
with a 0.5-in. throat diameter to the larger
size nozzles used in recoilless rifles, a
similitude relation in nozzle erosion is
required. For two nozzles of throat diameters
D { and Z) 2 , and working with the same
pressure-time curve it follows, from the
Boelter-Dittus equation for forced convection
heat transfer in a pipe, that
6-36
AMCP 706-238
h = / 5 iY /5 (6-41;
h \dJ
where hi and h 2 are tin respective heat
transfer coefficients. Denoting the fractional
increase in the throat area by € x = 2(A D x )/D l
and e 2 = 2(AZ) 2 )/Z) 2 , and assuming the
respective increases in throat diameters to be
proportional to the surface conductance, one
finally gets
On the basis of this relationship the erosion of
nozzles of recoilless rifles with bore diameters
from 15 mm to 381 mm (15 in.) have been
estimated for various materials, and the
results are shown in Table 6-5 for the case of
the ratio of the bore area to nozzle throat
area being equal to 1.5 (Ref. 17). Further¬
more, erosion data given in Table 6-5
correspond to a duration of gas flow of about
10 msec, the average chamber pressure during
the cycle being approximately 20,000 psi. For
longer duration of gas flow at the same
chamber pressure and gas temperature, the
erosion Figure in Table 6-5 must be multiplied
by a correction factor. Based on heat
conduction theory, it is suggested that for
nozzles of less than 5 in. in throat diameter
this factor be the time ratio, and for nozzles
having a throat diameter greater than 5 in. the
multiplying factor be the square of the time
ratio in order to estimate erosion for flow
duration longer than 10 msec.
6-17 OTHER FACTORS THAT AFFECT
EROSION RATE
The effects of other factors on nozzle
erosion are discussed in Ref. 17. These
include such factors as the isochoric flame
temperature of the propeilant, the shape of
the nozzle, the initial temperature of the
nozzle, and the test history. A summary of
these effects follows:
1 . Effect of Isochoric Flame Temperature.
The three types of propellant-Ml, M10, and
M2-used to study the effect of propellant
flame temperature on the rate of erosion of
metals have the isochoric flame temperatures
of 258C°K, 3010°K, and 3540°K, respective¬
ly. Results of the tests with seven metals
indicate (Ref. 17) that, in general, erosion is
reduced appreciably through the uee of a
propellant with a lower flame temperature.
One exception to this is found in the tests
with cast steel (approximately SAE-4340) for
which the erosion with the M1 Propellant was
almost identical to that with the M10
Propellant. This has been confirmed theoret¬
ically by an analysis based on considerations
of hear transfer and surface melting. The
results show that a combination of conditions
can exist under which the rate of erosion will
be the same despite the different flame
temperatures.
2. Effect of Nozzle Snape. Four nozzle
shape characteristics were investigated (Ref.
17) to determine their relative effects on
erosion rate. The results are
a. The erosion rate of nozzles with
cylindrical sections at the throat 0.25 in.,
0.50 in., and 0.91 in. long showed that
cylindrical throat sections of the lengths
considered had no appreciable effect in terms
of throat area increase when compared with
standard test nozzles of the same material
(SAE-4150).
b. Tests with a nozzle having a square cross
section indicate that no discernible erosion
takes place at the corners. As anticipated
from gas flow and heat transfer patterns,
sharp corners have practically no effect on
erosion.
c. In order to quantitatively compare
oblong nozzles with circular nozzles of the
same cross-sectional area, oblong nozzles with
cross sections having length-to-width ratios
equal to 1.875 and 4.33 exhibited a spread of
less than one percent in the measured
6-37
AMCP 706*238
(
TABLF 6-6
ESTIMATED EROSION OF GUN NOZZLES AS A FUNCTION OF BORE
DIAMETER (A b /A t - 1.5) AT 30,000 p$i MAX PRESSURE
Erosion par 100 Rounds
(pmnf throat an incraasa)
0 B \
D a \
/DA 6 *
Gun
Cupaloy and
mm
in.
in.
U/
SuN
EJIuloy A
Tantalum
15
0.59
0.482
1.0460
62.8
12.10
23.40
30
1.18
0.963
0.4560
27.3
5.30
10.20
45
1.77
1.448
0.2790
16.7
3.20
6.30
60
2.36
1.936
0.1980
11.9
2.30
4.40
75
2.95
2.410
0.1510
9.1
1.75
3.40
105
4.13
3.375
0.1010
6.1
1.20
2.20
150
5.91
4.820
0.0660
4.0
0.77
1.50
200
7.87
6.390
0.0463
2.0
0.54
1.00
250
9.85
8.050
0.0357
2.1
0.42
0.80
300
11.80
9.620
0.0290
1.7
0.34
0.66
381
15.00
12.250
0.0215
1.3
0.25
0.50
• - bor* diamottf of rifle; O t - throat diamtter of nozzle
At 30,000 pci (Ml0 Propellent) erosion of cast molybdenum, sintered molybdenum (sp gr 10.0), or molybdenum copper (sin¬
tered) should be negligible for ail sizes in Table 6-5.
increases in throat area after 50 rounds at
15,000 psi with the M10 Propellant. The cross
section shape does not appear to have a
critical effect on the erosion rate.
d. Tests with nozzles having divergence
angles of 7.5 deg, 15 deg, and 30 deg indicate
that the divergence angle has no appreciable
effect on the conditions of gas flow affecting
the rate of erosion at chamber pressures of up
to 15,000 psi.
3. Effect of Initial Temperature. In a
recoilless rifle with a high cyclic rate of firing,
the effect of an elevated initial temperature
on the erosion of nozzles is expected to be
appreciable. Tests have shown that when the
initial temperature is 225°C, the erosion of
gun steel at 30,000 psi (M2 Propellant) is
twice as high as the erosion at an initial
temperature of 25°C, as predicted on the
basis of heat transfer and surface melting
considerations.
4. Effect of Test History. To study t,ie
effect of test history on subsequent rate of
erosion, three cast steel nozzles using the Ml0
Propellant were tested. Fuing on one was
started at the 10,000 psi level while the firing
on another was begun at the 20,000 psi level.
The subsequent rates of erosion of these
nozzles at 35,000 psi were compared with the
erosion rate of the third nozzle on which
testing was initiated at 35,000 psi. All were
found to erode at very nearly the same rate.
This indicates that the lower pressure portion
of the firing schedule is not a severe test for
the cast steel, and it is to be expected that the
subsequent rates of erosion of other metals
superior to gun steel at the higher pressure
levels would not be significantly affected
cither. This was shown also to be true for cast
molybdenum tested with the M2 Propellant.
No general conclusion applicable to other
metals may be drawn on the basis of these
results alone.
6-38
AMCP 70*231
SECTION V
BORE-SIZE NOZZLE
During the course of study (Ref. 15) of the
feasibility of a recoilless rifle that permits rear
loading without a breech mechanism, the S7
mm Recoilless Rifle, Ml 8, was modified to
have a concentrically mounted nozzle that
simply consisted of a straight pipe of uniform
cross section with an inside area of about 1.02
times the bore area of the rifle. The study was
initiated in recognition of the obvious
practical advantages of the straight-pipe
nozzle over the conventional de Laval nozzle.
These advantages are simplicity; ease of
fabrication, which is of greater importance
when the nozzle must be replaced after
erosion has changed rifle performance beyond
a desired level; and the elimination of a costly
breech mechanism.
On the basis of the experimental data
obtained, it is concluded that a perforated
pipe of length less than S in. will meet the
requirements of negligible recoil imbalance
with reasonable ballistic performance. In the
event that a higher ballistic efficiency is
desired, the bore-size system of internal rings
and external perforated pipe can be used and,
with the application of a combustible
cartridge case and the improvement of
ignition conditions to diminish the propellant
loss, the total charge required can be reduced
considerably.
Use of several types of the bore-size nozzle,
including perforated and nonperforated pipes
as well as combinations of externally located
pipes with internally located rings, was made
in the series of tests conducted.
The first series of tests consisted of firing
the M18 Rifle with a 4 in. long straight pipe
located externally at the rear of the chamber.
Ihe recoil imbalance recorded v'as approxi¬
mately 16 percent in the forward direction.
This correlates well with the theory when a
nozzle discharge coefficient of 0.72 is
incorporated to account for the further
decrease of the stagnation pressure and mass
flow rate at the sharp-edged entrance of the
straight-pipe nozzle (Ref. 15). In order to
obtain a balanced rifle with a bore-size nozzle,
the forward recoil imbalance was reduced by
diverting a portion of the rearward momen¬
tum of the gases away from the rifle axis,
thereby reducing the momentum transfer to
the rifle. This was accomplished in two ways.
One, by adding a device similar to a “muzzle
brake” to the end of the straight pipe; and
two, by perforating the wall of the straight
pipe. In both of these systems, the amount of
momentum change is a function of the
amount of gas diverted and the angle ol
diversion. Experimental data indicate that the
effect of the brake is greater if the slot width
is increased to gain bleed-off area, compared
to simply increasing the number of slots,
when the wider slots are used. Also, the
influence of the “nozzle brake” on the
interior oallistics of the rifle is negligible, so
that the net effect is to reduce the forward
thrust on the rifle without measurable effect
on the mass flow entering the straight-pipe
nozzle.
As to the use of the perforated, straight-
pipe nozzle with perforations at 90 deg to the
axis, an obvious disadvantage is the spraying
of hot gases in the immediate region normal
to the rifle axis. A conical shield or deflector
employed as a device to deflect the bleed-off
gas flow toward the rear of the rifle did not
prove to be satisfactory because it gave
unacceptable recoil imbalance. Consequently,
a bore-size nozzle was designed with perfora¬
tions at angles of 45 deg and 65 deg to the
bore axis. Fa^tax pictures indicated qualita¬
tively that there is less eddying of gases into
6-39
AMCP 70*238
the gunnels area with the holes at 65 deg to
the bore axis than with the holes at 90 deg.
A straight-pipe, sharp-edged entrance noz¬
zle with an expansion cone was tested in the
differential thrust bomb for comparison with
a straight-pipe, sharp-edged entrance nozzle
without an expansion cone. The net differ
ence of thrust imbalance between the two
nozzles was about 20 percent, which is 7
percent less (in the forward direction) than
the value predicted by the theory. The
difference is attributed to decreased thrust
efficiency of the nozzle due to a discontinuity
of surface slope, which could cause formation
of oblique shock wa v es and vortex flow, with
resulting loss of forward thrust.
In the series of tests employing straight-
pipe nozzles located internally, several other
types of bore-size nozzles were employed for
location inside the chamber of the M18 Rifle.
This was an effort to achieve mass flow
control by effecting th? dynamic flow
contraction of a main gas stream by the
introduction of a second high-velocity gas
stream at right angles to the main stream.
Thus, the net effect is, in general, entropy
increase, or total pressure decrease of the
combust l on gases in the nozzle. When
compared with the results of the externally
located, perforated, bore-size nozzles, the
ballistic results are, in general, favorable.
However, in order to maintain a balanced
rifle, it was found necessary to use the
externally located straight-pipe nozzle with
65-deg perforations in combination with these
internal devices. As a result of the higher
propellant loss with these systems, it was
obvious that the ballistic efficiency of the
Ml8 Rifle was lowered by the use of a
bore-size nozzle, as compared with its
operation -with a standard convergent-
divergent type of nozzle. In the event that
higher ballistic efficiency is desired, the
bore-size system of internal rings and external
perforated pipe could be used and, with the
application of a combustible cartridge case
and the improvement of the ignition condi¬
tions to reduce the propellant loss, the
required amount of charge could be reduced
considerably.
Lastly, in connection with the erosion rate
of the straight-pipe, bore-size nozzle with
sharp-edged entrance, investigations per¬
formed in the vented bomb indicated the
eroding of the entrance to a radial contour
shape and the uniform erosion of the inside
pipe surface over the full kngth. The total
erosion rate was about one-sixth that of a
convergent-divergent nozzle throat and there
was little change in the thiust of the nozzle
when tested in the differential thrust
bomb-in good agreement with the theoretical
prediction when the very low expansion ratio
of the nozzle is taken into account.
AMCP 706-238
SECTION VI
RECOIL COMPENSATORS
The effectiveness of recoilless rifles can be
increased considerably if the effects of nozzle
erosion on rifle performance could be
com* ensated for continuously during the life
of the rifle. This is achieved by use of recoil
compensators for nozzle erosion.
It is reported (Ref. IS) that during the
investigation of the effects of chamber
configuration and nozzle approach area on
rifle performance, tests were performed
whereby the average muzzle velocity was
maintained approximately constant by varia¬
tion of the charge weight, it is noted that* hid
the charge weight remained constant during
these tests, muzzle velocity and chamber
pressure could have been maintained constant
by enlarging the nozzle throat, which would
also have brought the rifle closer to a
balanced condition. Thus, these tests indi¬
cated that compensation by means of flow
throttling was entirely practicable; however, a
mechanical set-up using a set of chamber
liners is impracticable. Rather, such throttling
devices as tested in the study of the internally
located bore-sized nozzles in the 57 mm Rifle,
Ml8, are mere practical if the chamber design
permits variable throttling by variation of the
axial position of the compensator from the
nozzle. The opportunity for this type of
correction would not exist if the gases were
not so much underexpanded in the produc¬
tion rifles.
Devices that have been designed and tested
in the 75 mm Rifle, M20. to compensate for
nozzle erosion are described in Ref. 15. These
devices were rings, of various cross-sectional
shapes and sizes, located at various positions
in front of the nozzle. The effects of size,
shape, and position on rifle imbalance and
projectile muzzle velocity were determined.
Tlie results indicate that the change of
percent imbalance in recoil is, in general,
greater than the percent change of the
projectile muzzle velocity. This effect is
greater with a ring of square cross section, a
ring of circular cross section having the least
effect. Test results also indicate that, with
rings of square cross section, the ring with a
larger cross section provides greater compen¬
sation throughout the entire range of axial
adjustment, and also, for an adjustment less
than 0.5 in. from the nozzle, its effect
increases at a faster rate. This generally
superior effect of the square ring with a larger
cross section is due to the greater restriction
of flow provided.
Studies of the devices to compen¬
sate for nozzle erosion were also conducted
with the 106 mm Recoilless Rifle. T170.
Results of tests reported (Ref. 15) indicate
that a conical recoil compensator in the 106
mm Rifle appears to be more efficient, since
the change of percent rifle imbalance is nearly
equal to the percent change of projectile
muzzle velocity. This is attributed to its
throttling of gas against a forward chamber
wall instead of the nozzle entrance, as in the
ring compensators.
Complete and detailed information on
rotational recoil compensators for nozzle
erosion is given in Ref. 15.
6-41
AP*CP 706-238
SECTION VII
BLAST EFFECTS
6-18 INTRODUCTION
The nozzle blast from recoilless rifles
contains several components that are poten¬
tially damaging or dangerous. These include:
the nozzle or back jet of high-velocity air and
gases; the unbumed propellant grains ejected
from the breech at high-velocity; the flame
illuminating the surrounding area; and the air
blast, i.e., the airborne shock wave or blast
pressure pulse.
When a recoilless rifle is fired, a very large
volume of propellant gas streams from the
nozzle a* supersonic speed and compresses the
surrounding atmosphere, thus initiating a
shock wave. The shock wave is bounded by an
extremely sharp front, caJled the shock front,
which represents a discontinuity in density,
pressure, and temperature of the atmosphere.
As the shock front continues to move
outwa* \ the peak pressure, the velocity of
propagation, and the impulse of (he shock
wave decrease while the transient duration of
the shock increases. When its velocity
decreases to me sonic level, the shock wave
becomes simply an impulse sound wave.
The phase where the pressure first rises
very sharply from atmospheric to a peak value
and xhen declines to the atmospheric level is
known as the positive or pressure phase of the
shock wave. Immediately after the pressure
phase, the pressure continues to decline to
subatmospherie levels and then returns to
normal. This second phase is known as the
negative or suction phase and lasts consider¬
ably longer than the positive phase, the peal,
negative pressure being only a fraction ol the
peak pressure of the positive phase.
The effectiveness of a blast wave may be
measured by the peak overpressure and the
impulse of the positive phase at various
distances from the origin of the blast. The
peak pressure is the pressure jump at >ne
shock front, which is the lughest pressure in
the shock wave. As a measure of both the
intensity of the pressure and its duration, the
impulse of the positive phase is of basic
importance. This positive impulse is equal to
the area under the pressure-time wave of the
positive phase, and is approximately equal to
one-half the peak pressure multiplied by the
duration of the positive phase. For highly
elastic structures, the positive impulse of the
wave will be effective; for brittle structures,
the damage is generally determined by the
peak overpressure.
6-19 VARIOUS DAMAGE MECHANISMS
In the design of a recoilless rifle, an
important factor to be considered is the
damaging effect of the buckblast ca personnel
and itructures behind the nozzle of the gun
and, since the early 19S0 », the characteristic
blast zone behind the reccilless rifle has been
the object of extensive analytical and
experimental investigations, with a view
toward determining blast danger zones and
establishing techniques of blast reduction or
diverting. Strictly analytical solutions to the
blast problem have been found inadequate in
determining the characteristics of the shock
structure, and a combined empirical-analytical
approach usually is employed (Ref. 18).
BIa?.t studies usually consist of the
experimental mapping of the blast pressure
field (Refs. 19, 20 y 21) and the determination
of the response of structures to the blast
loadings. The objective of the first part of the
study of blast ss to determire the degree to
which recoilless rifle and ammunition design
can influence the blast envelope in terms of
AM&706-2M
both intensity and location of the overpres¬
sure. The second part of the study treats the
effects of the blast overpressure on nearby
objects, both structurally and from a control
aspect, in order to explore the possibility of
optimizing the rifle for both zero recoil and
minimum blast effects. However, a minimum
blast Held requirement may not be realized by
designing breech nozzles for a single steady-
state flow condition obtained for the single
set of values of chamber pressure, nozzle
expansion ratio, and nozzle length that will
balance the total impulse. In order to
optimize nozzle design for both recoil balance
and minimum blast field effects, a detailed
evaluation of the time-dependent flow f.eld
may be required. To date, such a study has
not been available in the literature.
In addition to structural damage due to the
blast from the nozzle of r. recoilless rifle, the
danger of physiological damage also exists.
The extent and nature of the damage caused
by the concussion produced by the blast wave
depend on the intensity of the blast, its
impulse, the position of the subject with
respect to the blast, angle of incidence of the
blast, and the presence of reflected blast
wave. At close range, the nozzle jet (hack jet)
is very destructive especially along the axis of
the gun. Generally, a significant amount of
unbumed propellant is also ejected during
backbiast, ranging in size from whole grains
down to small slivers. This is a most serious
component of backbiast danger, since its
irJ^sile effect extends over wider angles and to
greater distances behind the rifle than any of
the other injuring factors. The amount of the
solid propellant ejected is a function of the
chamber, nozzle and cartridge case designs,
the ignition of the propellant, and the
pressure level at which the rifle operates; the
nigher the operating pressure, the smaller the
percentage quantity of unbumed propellant
ejected. Inasmuch as the back jet and
propellant missile effects are severe in regions
where the flame causes only the minor
damage of singeing the hair, the flame
component of backbiast appears to be
negligible. The blast-type injury caused by
high pressures associated with the back jet
usually occurs only ir. regions where the
effects of the beck jet and ejected propellant
are very large so that the blast wave pressure
factor is overridden in severity by the other
two. Consequently, the pressure wave is not a
primary source of danger to life, but is a
possible complication added to the other
already severe injury mechanisms. Injuries to
the lungs and the auditory mechanisms (Ref.
29), and damage to the nervous system are
the most common und easily produced effects
of the blast pressure. Detailed information on
the effects of nozzle blasts is contained in the
experimental investigation of Ref. 22. Other,
less frequently occurring, blast induced
injuries have been reported by the National
Research Council in Ref. 23.
Apart from causing physiological effects,
blast from recoilless rifles also creates
psychological factors which may be signifi¬
cant. Ref. 23 reoorts that blast intensities that
are too small to induce serious physiological
disruption often cause psychological effects
of extreme lethargy and feelings of fatigue. It
is not possible to derive concrete conclusions
that define the limits of blast pressure which
can be tolerated by humans and animals;
there is much disagreement on these limits
since there have been only very few
experimental studies conducted under con¬
trolled conditions.
6-20 BLAST AND FLASH PATTERNS
Since the early stages of the development
of the rccoilless rifle, backbiast has been
considered the prinipal objectionable by¬
product of the recoil-balancing nozzle. The
tremendous blast that results from the escape
of propellant gas to the rear of the rifle is an
inherent characteristic of the design of the
weapon, and constitutes the main drawback
to the use of recoilless rifles. The magnitude
of the blast is determined by the projectile
energy, and studies have indicated that it is
incapable of considerable reduction. Person¬
nel and materiel therefore must be adequately
protected for many yards to the rear. In the
644
AMCP 706-238
event of the 57 mm rifle, for example, the
danger zone is a cone extending 50 ft to the
rear and 40 ft wide at its widest point.
Because of the danger of flying particles
thrown up by the blast action, personnel
within 100 ft of the rear of the breech must
not face the weapon. Another, but much less
serious, effect of the blast is that it may be
the means of enemy observation of battery
locations. Obviously, if would be desirable to
have this disadvantage reduced or eliminated.
Studies have indicated that the backblast
levels developed by a rccoilless rifle can be
reduced by firing at lowered maximum
operating pressures. A propellant grain that
provides a high piezometric efficiency (neu¬
tral pressure-time characteristic) will permit
attaining a high muzzle velocity with the
lowest maximum operating chamber pressure.
The structure of the high-velocity nozzle
jet and the major factors influencing it aie
discussed extensively in the experimental and
theoretical studies reported in Ref. 24. These
studies pertain to axisymmeiric free jets
exhausting from sonic and supersonic nozzles
into still air and into supersonic streams. For
jets exhausting info still air, Jhe primary
variables considered are jet Mach number,
nozzle divergence angle a, jet pressure ratio
P e /Pa> and the ratio of specific heats of the
jet. The effects of most of these variables
upon jet structure, primary wavelength, and
the shape and curvature of the jet boundary
are studied in Ref. 24. The gaseous jet
exhausting supersonically into still air has
been known to exhibit a periodic or chain-like
structure at least as early as the observations
of Lord Rayleigh in 1879. Since then, several
theories have been advanced for the predic¬
tion of the primary wavelength (length of the
first periodic segment of the jet) and the
secondary wavelength (length of the succeed¬
ing periodic segments) which is known to
differ from the primary, particularly at the
higher jet pressure ratios*. At a constant value
of PelPu an <3 o, the experimental results show:
that the primary wavelength increases with|
increasing Mach number. Divergence angle a is
indicated to be of secondary importance
within the range from 0 deg to 20 deg. When
a jet exhausts from a nozzle into still air, it
will undergo a two-dimensional expansion if
P e /Pa > 1- The amount of this expansion can
be measured by the resulting initial inclina¬
tion of the jet boundary. In the event of
extreme jet pressure ratios, results indicate
that large initial inclinations will occur.
Preliminary calculations based on the method
of characteristics to determine the shape of
the jet boundary and to observe the
formation of the jet structure indicate that
the effect of increasing a is to promote the
formation of shocks within the jet. The effect
of increasing y from 1.2 to 1.4 is also to favor
the formation of shocks within the jet.
Approximately 3,000 calculated boundaries
are presented which cover p e /p a ratios from 1
to about 42,000; values of 7 of the jet from
1.115 to 1.667; values of a from 0 deg to 20
deg; and the jet Mach numbers from 1.0 to
3.0. Most of these boundaries (2,960)
comprise a systematic study in the jet
pressure ratio range from 1 to 10 for conically
divergent nozzles.
Typical curves of the calculated jet
boundaries for a jet Mach number M e of 2.0
and 7 - 1.2 for the jet are presented in Fig.
6-11 fora = 5 deg, and in Fig. 6-12 fora = 10
deg. In these curves, x is the abscissa and y is
the ordinate of a point on the boundary of
the jet, and r is the radius of the exit section
of the nozzle; the origin of the coordinate
axes being at the center of the nozzle exit
section. The effect of increasing the jet
pressure ratio is obvious from these figures.
Effects of the other variables at a jet pressure
ratio of 5 (arbitrarily chosen) are shown in
Fig. 6-13. The effect of increasing M c is to
decrease the initial inclination of the boun¬
dary, to increase the maximum diameter of
the free jet, and to move the maximum
diameter farther away from the plane of the
jet exit. The effect of increasing 7 is to
decrease the initial inclination of the boun¬
dary, to decrease the maximum diameter of
the free jet, and to move the maximum
diameter closer to the plane of the jet exit.
AMCP 706*238
Inset ordinate scale it twice
2
r
Figure 6-11. Jet Boundaries for Jet Pressure Ratios from 1 to 10
(u = 5deg,y= 1.2,M^2.0)
The effect of increasing the nozzle divergence
angle a is to increase the initial inclination of
the boundary, to increase the maximum
diameter of the free jet, and to move the
maximum diameter closer to the plane of the
jet exit. These effects appear to be typical of
all other jet pressure ratios. It is also apparent
that, for a particular value of y of the jet,
there are a number of combinations of M e ,a ,
and p e lp a which produce essentially the same
boundary. At large values of p e lp a , the
calculated jet boundaries also are presented in
Insst ordinals scale Is
Figure 6-12. Jet Boundaries for Jet Pressure Ratios from 1 to 10
(a = 10 deg, y = 1.2, M, * 2.0)
6-46
AMCf» 70*238
(A) Effect of M e
(B) Effect of 7
r
(C) Effect of a
Figure 6-13. Jet Boundary Patterns for Various Parameters (Example of the effects of
jet Mach number, ratio of specific heats of the jet, and nozzle divergence
angle upon the shape of the jet boundary, p 8 /p t * 5)
Ref. 24 for values of a and y larger than those
shown in Figs. 6-11 and 6-12. The enormous
size that the free jet may attain at large ratios
of jet pressure is apparent, particularly for
7 = 1.20, when the initial inclination of the
boundary approaches and exceeds 90 deg.
Indicated also is the elimination of the jet
shock with increasing p e /p a at large values of
the latter.
The analytical method of Ref. 18 for
predicting the backblast overpressure field to
6-47
AMCP 700-2311
the rear of the recoilless rifle is based on the
assumption that the solution of the spherical¬
ly symmetric blast problem due to a point
source can be made applicable to the nozzle
blast problem by incorporating an empirical
factor to account for the directional effect of
the nozzle gases. This factor, called the
directional coefficient, is a function of the
angle included by the rifle axis and the line
joining the center point of the nozzle exit
plane to the point under consideration, and is
given in Ref. 18. While this procedure does, in
general, correlate reasonably well with the
experimental results of nozzle blast studies, it
is not applicable at positions close to the
nozzle (less than 5 ft away from nozzle)
where the shock is only partially formed and
the flow apparently is undefined.
Recent observations of a series of high¬
speed movies indicate that the behavior of the
overall nozzle blast phenomenon associated
with the firing of standard recoilless rifles is as
follows. The initial shock front, driven by the
hot gas to cold air contact surface, emanates
from the breech nozzle exit and appears to
diffract about the nozzle exterior, forming a
bubble which expands and translates aft. The
bubble is rendered partially visible due to the
intense radiation field directed to the aft. The
radiation is scattered by multicomponent and
multiphase efflux, becoming mom isotropic
and less intense with increasing efflux density
and distance from the exit plane of the
nozzle. It is concluded that:
1. The initial shock front from a recoilless
rifle emanates from a finite “line-source” that
directs the blast so that it does not possess
spherical symmetry. The classical, “point-
source” blast vave theory does not allow for
directivity of the blast field nor for the
continuous addition of energy to the blast
field produced by such a source.
2. The arrival of shock phenomena as
measured by pressure transducers located or. a
plane containing the axis of the rifle yields a
Mach number distribution tliat builds up to
648
Mach 2 intensity before decreasing aft to
transonic intensities.
3. The existing data indicate that overall
backbla.it damage may be due to a composite
of two recoilless rifle overpressures consisting
of the initial shock front witn its reflected
overpressures and the quasi steady-state jet
plume impingement overpressures.
Another important and objectionable
by-product of the recoil-balancing nozzle of a
recoilless rifle is the nozzle flash that results
from the luminosity of the hot gases issuing
from the nozzle. Muzzle flash in recoilless
rifles is negligible in comparison with nozzle
flash which is of much greater intensity and
longer duration. For example, the flash from
the nozzle of the 75 mm M20 Recoilless
Rifle has an integrated intensity of the order
of 10 4 candle-seconds. Its shape is approxi¬
mately that of an ellipsoid of revolution with
a diameter of about 10 ft and a major axis of
about 30 ft. Obviously, nozzle flash is
detrimental from the standpoint of detection
by the enemy and, to a lesser extent, due to
the d.mger of impairment of vision of the user
of the rifle. If adequate flash suppression
measures are not taken, the flash problem can
become very serious.
While the mechanism of flash is not
understood in complete detail, the long
duration of nozzle flash has enabled the
examination of its structure and time-develop¬
ment in greater detail than has been possible
in the case of conventional gun flash. The
details have been recorded by means of
high-speed motion pictures. Studies of the
flash structure over a range of variables—in¬
cluding nozzle shape, chamber pressure, and
propellant composition --have led to a clearer
understanding of the mechanism of the flash
and the methods of reducing it. Qualitative
results indicate that, for a given chamber
pressure, the flash decreases as expansion
ratio increases and, for a given expansion
ratio, flash decreases with chamber pressure
increase (sec Ref. 14).
■g- ‘ I h.'l I J lohWlttfa'Afo i"W£rki/ ;•
AMCP 70*23*
In the study of flash, three regions of
luminosity have been observed. As the
propellant gases exit from the nozzle of a
recoilless rifle, they are sufficiently hot to be
self-luminous and constitute a small region of
low luminosity at the nozzle. This is called
the primary flash. Upon exit from the nozzle,
the gases rapidly expand and cool and the
luminosity disappears, forming the dark zone.
Here the gases are overexpanded and subse¬
quently are recompressed udiabatically
through a shock. This process raises the
temperature to a level almost equal to that of
the nozzle exit temperature, so that the gases
are again luminous and form the intermediate
flash. In the meantime, the gases have
entrained air, and a combustible mixture has
been fonned of the unbumed hydrogen and
carbon monoxide in the nozzle gases. When
the recompression process raises the tempera¬
ture of this mixture above its ignition level,
the combustible mixture will ignite and bum
as a diffusion flame constituting the secon¬
dary flash.
Such metallic impurities as sodium, potassi¬
um, and calcium-which are always present in
propeJlants-are responsible for most of the
luminosity in flash. However, it does not seem
possible to eliminate flash by eliminating
these impurities. It has been found that flash
can be greatly reduced by cither the addition
of such salts of potassium as iodide, bromide,
oxalate, and sulfaU to the propellant, or by
use of a mechanical device on the nozzle.
Chemical suppressors act by inhibiting com¬
bustion in the secondary flash and hsnee
suppress only the secondary flash. Mechanical
suppressors reduce the intensity of the shocks
and therefore suppress both the intermediate
and the secondary flash. Extensive treatment
of gun flash and its suppression can be found
in Refs. 25, 28, and 29.
6-21 EXPERIMENTAL DATA
6-21.1 PRESSURE CONTOURS
The blast field behind several types of
recoilless rifles has been experimentally
determined (Refs. 18, 19, 20).
In the experimental study reported in Ref.
18, the 57 mm Rifle M18, the M18 with
central nozzle adapter, the 75 mm Rifle T21,
and the 105 min Rifle M27, were fired and
the peak pressures to the rear of these rifles
were measured at angle., ranging from zero to
70 deg from the nozzle axis and at distances
up to approximately 22 ft from the nozzle
exit. The results of these field tests indicate,
generally, that the pressures build up to a
peak at about 4 or 5 ft from the nozzle exit
followed by quasi-steady-state pressures there¬
after. Until the maximum peak pressure is
leached, i.e., at positions close to the nozzle
(less than 5 ft away from nozzle), the flow
phenomenon appears to be undefined for all
the rifles fired. The major experimental
problem in these pressure measurements is in
the design of pressure gages which can
withstand the blast without damage.
The experimental results indicate (Ref. 18)
that, while the standard 57 mm Rifle M18
yields a maximum peak overpressure of 41 psi
at 4 ft from the nozzle exit along the axis of
the nozzle, the corresponding maximum peak,
overpressure in the event of the central-nozzle
adaptation of the Ml8 Rifle is 14 psi at the
same distance. The standard 57 mm rifle
bums approximately 0.76 lb of charge
producing a peak chamber pressure of about
6500 psi, while the central-nozzle 57 run nile
bums about 0.67 lb at a peak chamber
pressure of approximately 4800 psi.
Extensive firing of the 75 mm Rifle T21
indicated (Ref. 18) that the maximum peak
overpressure to the rear is approximately 50
psi at 4 to 5 ft away from the nozzle, and that
the pressure wave along the nozzle axis
fluctuates considerably. The charge weight for
the rifle was 2.90 lb.
The experimental results (Ref. 18) ob¬
tained from the 105 mm Rifle M27 show that
the corresponding peak overpressures are
649
AMCP70S42S
lower than those obtained from the 75 mm
Rifle T21. lit particular, the maximum peak
overpressure measured from the 10S mm rifle
is about 25 psl at 7 to 8 ft from the nozzle,
while that from the 75 mm rifle is
approximately M psi at about the same
distance. The charge weight for the M27 was
7.9 lb and for the T21 was 2.9 lb. The
number of the nozzles for each rifle was 4.
Fig. 6-14, given in Ref. 20, is a contour map
of the peak pressures for the backblast of the
105 mm M27 Recoilless Rifle. It is seen from
this figure that the pressures behind the M27
are very high at distances of less than 15 ft
from the breech, near the line of tire. Twelve
feet to either side of the line of fire and to
distances greater than 35 ft back, the
pressures rise to more than 5 psi above
ambient. For pressures greeter than 10 psi,
these distances are 5 ft to either side of the
line of fire and 30 ft along the rifle axis.
Correlation of the wide range of experi¬
mental results of such blast studies with a
theoretical model, which is based on shock
sphere energy (and, ultimately, on chamber
pressure and nozzle size) and which also
includes a factor to account for the effect of
directivity, appears to be reasonably good
except in regions close to the nozzle (see Ref.
18).
6-50
AMO 7C!-23t
6-21.2 DANGER AREAS
Both personnel and materiel in the
immediate vicinity of a recoilless rifle must be
protected from the dangerous effects of the
blast from its nozzle. Results of an experi¬
mental investigation of the physiological
effects of such blasts on goats have been
reported in Ref. 22. The following are the
conclusions of this investigation.
1. The danger areas behind reccilless rifles
have roughly a tear-drop shape; the danger
decreases as one goes farther behind the gun
and farther off the gun axis.
2. The maximum backward extent of the
danger zone occurs on the gun axis and
extends to about 30 ft for the 57 mm and to
about 80 ft for the 75 mm and the 105 mm
rifles.
3. At its widest extent, the danger area
reaches about 8 ft on either side of the axis
for the 57 mm rifle and about 15 ft on either
side of the axis for the 75 mm and the 105
mm rifles.
4. The most important single injuring
factor is the missile effect of unbumed
propellant expelled at high velocity from the
breech of the rifle, and it is this factor that
determines the maximum extent of the
danger areas.
5. The back jet of high velocity air and
gases is very dangerous within about 10 deg
either side of the gun axis up to distances of
about 30 ft for the 75 mm and the 105 mm
rifles, and up to about 10 ft for the 57 mm
rifle.
6 . During backblast, conditions are prob¬
ably incompatible with life on the axis of the
rifle within about 5 ft of the 57 mm rifle and
within about 20 ft of the 75 nun and the 105
mm rifles. Here, back jet and propellant
missile effects combine to cause extremely
serious injury or death.
7. Improvement in efficiency of propellant
burning should result in reduction of the
extent of the danger areas.
8 . Flame (flash) present in backblast
presents an extremely minor hazard for the
animal body, whether clothed or unclothed.
9. Blast pressure waves may cause injury to
the unprotected ear even in those locations
near the rifle that are outside the danger areas
found in Ref. 22.
10. The danger region extends from the
ground to a height of at least 6 ft.
11. The danger areas for the 75 mm and
the 105 mm recoillcss rifles are closely similar
in extent. The danger zones for the 57 mm
rifle extend over a region the dimension of
which is about half as large in any direction as
the corresponding ones for the two larger
caliber rifles.
It is recommended that attempts be
continued to improve the efficiency of
burning of the propellant since unbumed
propellant is the most important single factor
in determining the maximum extent of danger
regions.
6-21.3 DUCTING
Apart from physiological damage due to
the blast from the nozz’; of a recoillcss rifle,
there is the danger of structural damage. In
certain recoilless rifle applications-e.g., in
such enclosed installations as aircraft and
helicopter fuselages, and tank turrets-it is
necessary to devise methods of conducting
and diverting the backblast of the nozzle gases
out of the inclosure. These methods usually
consist of the channelling of the nozzle gases
through metal ducts and are known as
ducting. The purpose of ducting is to divert
the flow and also tend to dissipate the shock
wave of the blast in such a way as to protect
personnel and structures in the immediate
area of the rifle. Ducting of the recoil
6-51
AMC? 70*231
(A) Y-exhaust
Figure 6-15. f vpicsi Ducting Configurations
compensating gases is employed also in
front-orifice type of recoilless rifles (Ref. 26)
that differ from the conventional type cf
rear-orifice rifles in that the propellant gases
used to achieve recoil balance are bled from
the forward end of the chamber, thus
providing for simple breech mechanisms as in
the case of the closed breech guns.
Typical ducting configurations include the
Y-cxhaust, the gooseneck exhaust, and the
reverse curve Y-cxhaust which are illustrated
in Fig. 6-15.
The principal effects of ducting are on the
recoil and performance of the rifle, and on
the structural integrity of the ducts. In the
event the gas flow from the nozzle into the
ducts L subsonic, the disturbances of the
ducts are propagated upshcam and, through a
time lag, are felt at the chamber. The result is
a direct influence of ducting on tne
pressure-time history of the rifle chamber.
This, in turn, affects the motion of the
projectile in the barrel and, ultimately, the
muzzle velocity.
in the event the gas flow through tne ducts
is supersonic, flow disturbances cannot be
propagated upstream and the ducts can only
influence the pressure-time history of the
chamber if shock waves develop and travel
toward the chamber and render the flow
subsonic. The prediction of the formation of
shock waves in ducted flow and that of the
influence of the subsonic duct flow, requiring
unsteady flow analyses, on rifle performance
are far from being developed in sufficiently
simplified form for handbook use. In order to
determine these factors and the effect of
ducting on recoil, use must be made of
rigorous gas dynamic analyses together with
experimental data. However, as a reasonable
guide, some simplified results for the Y-duct
configuration given in Ref. 26 can be used to
determine the effect of ducting on recoil.
Obviously, a minimum quantity of gases need
to be bled from the rifle if the forks of the
Y-ducting are parallel to the rifle axis. In the
event of an included angle between the forks,
an additional amount of gases is required to
be bled off in order to insure recoillessness of
the rifle. These additional gases come from an
increase in the amount of the propellant sed.
Table 6-6 (see Ref. 26) shows the p, cent
increase in the weights of ammunition and
6-52
AMCP 706*236
TABLE *4
INCREASE IN CHARGE AND AMMUNITION
WEIGNTS-Y-DUCT COMPARED WITH
CASE OF ZERO INCLUDED ANGLE
IncriMi in
Amntuni-
Included Angle,
deg
Inoreeeeta
Ctarge Weight, %
don Weight,
%
0
0
0
10
1.1
0.3
20
3.7
1.4
30
0.1
3.4
charge asc iated with the Y-duct as com¬
pared with the case in which the included
angle is zero*
The structural integrity of the duct is
determined by the geometry of the duct and
the characteristics of the ammunition interior
ballistics. In addition to the possibility of
bursting due to high pressures, a curved duct
can be subjected to moderate tensile and
shear loads, and large bending moments as a
result of the centrifugal action of the mass
flow through the duct and has the tendency
to straighten itself out. This holds true
whether the curved duct is composed of
continuously smooth or piecewise smooth
curves. In general, evaluation of the dynamic
loading in curved ducts is a difficult problem
because the flow is of a complicated nature.
In addition to the axial flow along the duct,
there arises i secondary flow in each plane of
the cross section of the duct, called
circulatory motion. This vortidty, the exis¬
tence of shock waves, and effects of viscosity
and of heat transfer contribute to the
complexity of the flow pattern. However, a
rough theory of the effects of the changes in
the direction of gas flow-which impose
centrifugal thrusts on the duct systems, thus
producing shearing, tensile, and bending
stresses-is outlined in Ref. 27 to deal with
the problems of loading and induced stresses
in the system. In this simplified procedure,
each of the components of the more complex
shapes- which may consist of such elements
as a nozzle, straight-pipe section, curved
sections, abrupt changes of direction, in*
creases in pipe sections, diverging pipes-is
considered separately with respect to the gas
thrusts introduced by each of the elements,
nd these separate pieces are then combined
10 obtain the load cn the entire system. The
stresses induced as a result of the loading due
to gas thrusts are determined at various
sections of interest by methods of the
elementary theory of strength of materials.
Some safety factor for the effects of impact
loading, shock compression, and nonstation¬
ary flow should be included.
Ref. *7 also gives illustrations of this
procedure in dealing with the stresses induced
in a gooseneck exhaust and a reverse curve
Y-exhaust, together with a discussion of a
Y-cxhaust design. It is concluded that the
level of these stresses can be very liigh. and
adequate structural strength must be incor¬
porated in the design of ducting.
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2. G. P. Sutton, Rocket Propulsion Ele¬
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3. C. R. Foster and F. B. Cowles,
Experimental Study of the Divergence
Angle Effect in Rocket-Motor Exhaust
Sozzles, Jet Propulsion Laboratory. Cali¬
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Report No. 20-134, Junuary 1952.
6-53
AMCP7M-2M
4. H. G. Krull, W. T. Beale and R. F.
Schmiedlin, Effect of Several Design
Variables on Internal Performance of
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1946
7. R. Sauer. Method of Characteristics of
Three-Dimensional Axially Symmetric
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6-54
AMCP 706-238
R-1145, August 1953, 18 pp., Pitman-
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AMCP 706-107, Engineering Design Hand¬
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Document No. 6184, Ordnance Engineer¬
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August 1961.
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Gun Gases Through Straight Channels
and Bends. Report No. R-860, July 1948,
26 pp., Pitman-Dunn Laboratories.
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28. AMCP 706-255. Engineering Design
Handbook, Spectral Characteristics Of
Muzzle Flush
29. AMCP 706-251. Engineering Design
Handbook. Muzzle Devices.
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Reports I through 12. Armour Research
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6-55
AMCP 706-238
R. 0. Meade and R. T. Eckenrode, Psycho-
logicaJ and Physiological Effects of Gun Blast
with Special Reference to Recoilless Rifles, A
Preliminary Literature Survey, Human Engi¬
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September 1955, 34 pp.
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Recoilless Guns in Enclosed Installations,
Report No. R-861, Frankford Arsenal, Phila-
delpnia, Pa., July 1948, 49 pp.
Recoilless Rifle Handbook (Unpublished),
Prepaied at Frankford Arsenal, Philadelphia,
Pa.
6-56
AMCP 706-238
CHAPTER 7
SYSTEM EFFECTIVENESS
7-0 LIST OF SYMBOLS
A
— presented area of target, in. 2
A v
= vulnerable area of target, in. 2
E(K)
= expected number of personnel
incapacitations within an area
target, dimensionless
La
= lethal area, in. 2
Ph
- total hit probability, Pn = p ¥ Pk »
dimensionless
Ph
= horizontal hit probability, di¬
mensionless
Py
= kill probability, dimensionless
Pk\h
- conditional probability of a kill
given a hit on target, dimension¬
less
Pv
=“ vertical hit probability, dimen¬
sionless
V = muzzle velocity, fps
w = angle of fall, deg
x = horizontal target coordinate, ft
y = vertical target coordinate, ft
xy = mean values of target coordinate,
ft
& h = mismatch between main and
spotting rifle in horizontal direc-
tion.lft
8 ^ - mismatch between main and
spotting rifle in vertical direc¬
tion, ft
q2 = total variance in horizontal direc¬
tion, ft 2
o 2 = fth variance of the n independent
*' sources of error in horiziontal
direction, ft 2
a 2 = total variance in vertical direc-
> tion, ft 2
P(x)
- Gaussian hit probability density
function in horizontal direction,
ft" 1
p(y)
= Gaussian hit probability density
function in vertical direction,
ft-i
p K (yy) = probability of kill function for a
hit at point (x.v), dimension!^
R - range, ft
a x
= ith variance of the n independent
sources of error in vertical
direction, ft 2
= standard deviation in horizontal
direction, ft
— standard deviation in vertical
direction, ft
= density of personnel targets with¬
in area A , in, -2
7-1
AMCP 706-238
SECTION I
INTRODUCTION
A comparison of similar weapon systems is
made by examining the effectiveness by
which each weapon system defeats a specific
type of target. There are several ways to
measure effectiveness, but in most cases it is
defined as the ratio of target damage to unit
cost. This chapter will discuss the manner in
which target damage (kill probability) is
determined. Unit cost usually is measured in
dollars for munitions expended per killed
target.
Single shot kill probability is defined as t’ e
product of hit probability and conditional
probability of kill given a hit, and can be
expressed as the probability that a particular
weapon system with a prescribed method of
employment will inflict a specified level of
di.nage on a particular target. The determina¬
tion of hitting and killing with a single round
will depend on making certain assumptions
and the type of target and gun-ammunition
combination employed.
7-3
Preceding page blank
AMO* 706*238
SECTION II
HIT PROBABILITY
7-1 GENERAL
Hit probability is defined as the probability
of a hit or hits on a target occurring out of a
given number of rounds fLed at the target and
thus, for a given weapon system, depends
only on the target size and the overall weapon
dispersions (scattering of shots due to
unavoidable variations). In determining the
probability of hitting a particular target, it is
assumed that the distribution of impact
points about the aiming point is a Gdussian
(normal) distribution. It also is assumed that
the vertical and horizontal dispersions are
statistically independent so that the probabili¬
ty density functions of a Gaussian distribu¬
tion in the horizontal and vertical directions
are (Ref. 1):
(horizontal), ft" 1 (7-1)
(vertical), ft" 1 (7-2)
where
x,y = mean values, ft
of, - variances, ft" 2
The variance o£ is the mean-square value of
x about the mean and is given by the
following equation:
o 2 = jQ (x - x) 2 p{x)dx, ft 2 (7^3)
The square root of the variance is the standard
deviation a.
The standard deviation for the weapon
system is usually a combination of various
sources of error. If is the variance of one
of n independent sources of error in the
horizontal direction, then the combined
standard deviation a x is
and, similarly, in the vertical coordinate o y
7-2 SOURCES OF ERROR
Sources c f errors that must be considered
in determining hit probability are described
briefly in the following paragraphs:
1 . Vertical jump. Vertical deflections
(jump) are caused by an upward flip of the
gun tube as it leacts to the motion of the
projectile while in the vicinity of leaving the
muzzle.
2. Lateral jump. Lateral jumps are the
horizontal deflections caused by the same
reactions of the gun tube to the projectile as
it leaves the gun muzzle.
3. Nonstandard conditions. Nonstandard
7-5
Preceding page blank
AMCP 70&23S
conditions are the variations u ambient
weather conditions-such as temperature, air
pressure, humidity, and wind.
4. Cant. Cant error is the horizontal
deflection of the projectile due to a leaning or
tilting to one side of the gun (i.e., the gun
does not elevate in the true vertical plane).
5. Determination of target range. In the
case of unaided, visual observations (no
rangefinder), estimation of the target ranges
causes deviations from the actual range.
6. Variation in muzzle velocity. Variation
in muzzle velocity caused by gun tube wear
and lot-to-lot variations in propellant perfor¬
mance result in vertical deviations in the
trajectory.
7. Zeroing error. Zeroing errors, cau%d
by both changing conditions during zeroing
and firing for effect and observation error in
zeroing, result in horizontal and vertical
variations. Variations in cant, jump, and
crosswind and range error are 'xamples of
changing conditions from round to round;
while observation errors include the errors in
estimating the center of impact of a group of
shots and the error resulting from the fact
that a finite number of rounds are used to
establish the center of impact.
8. Gun laying error. Inability of the
gunner to place the crosshairs of his sight
precisely on the center of target results in gun
lay.ng errors.
9. Wind deflection. Wind deflection that
results from day-to-day variation in wind.
10. Row ld-to-round dispersion. Rrund-to-
round dispersion is the irreducible residual
error that remains.
7-3 CALCULATION OF HIT PROBABILITY
7-3.1 GENERAL
Based on independence of the lateral and
vertical dispersions and a rectangular target,
the equation for calculating the total hit
probability p H is
P u =P v P t: (7-6)
where
= vertical hit probability
Pk • lateral hit probability
The overall error in each of the lateral and
vertical directions in the target plane is then
the square root of the sum of the individual
component errors in their respective direc¬
tions.
In order to calculate H* orobability, certain
assumptions have to . made as to what
sources of error arc present and how they are
distributed for the specific weapon system.
Sources of error as discussed in par. 7-2 can
be categorized into either fixed biases,
variable biases, or random errors:
1. Fixed biases. Defined as those errors
inherent in a specific gun-ammunition design
and implies that the nature and characteristic
of the fixed bias are known. While a fixed bias
may vary with target range, it is constant for a
given target range and does not vary from
occasion to occasion or round to round. An
example of a fixed bias would be projectile
drift.
2. Variable biases. Defined as those errors
whose magnitudes are constant from round to
round during a given firing but vary from
occasion to occasion, such as crosswind and
cant.
7-6
AMCP 706-23S
3. Random e.tors. Defined as those errors
that vary from round to round during a given
firiug, gunner aiming being a prime example.
7-12 ERRORS ASSOCIATED WITH TYPE
OF FIRE CONTROL SYSTEM
As to the types of fire control that are
employed in the weapon system, recoilless
rifles can be divided into two groups-either
with or without a spotting rifle. Depending
upon the type of fire control used, certain
assumptions are made in <.rder to perform the
necessary error analysis.
1. Without Spotting Rifle. The total
standard deviations fer lateral and vertic al
directions of the mounted recoilless rifle
without a spotting rifle become
cr^total) = + + °l w + «i z
w- 7 ’
where
= (o\ + + <7? > <7?. +
*Z \ X FC X ZRBU U *c ” X ZJM W
<^(total) = (o^ + <^+0^
+ <4 +a2 yju>) m (7 " 8)
wheie
(T. = (c& +0? + (7?.
*Z \ y /c y ZRIM *ZJM
+ °yjuf +CT vjrvw) 1/2
Table 7-1 gives numerical values and defini¬
tions for the individual component errors for
recoilless rifles without spotting rifles. The
numerical values listed in Table 7-1 are
believed to be descriptive of actual firing
conditions. This group of error sources is
fairly complete for describing first round
accuracy, and the probability of hitting which
is premised on the combination of these
errors should be realistic (Ref. 2). Each error
source of Table 7-1 is assumed to occur at
random and follows a normal distribution.
2. With Spotting Rifle. Inherent in the
employment of a spotting rifle on a recoilless
weapon is the elimination of errors due to
ranging, and the addition of jump variation,
muzzle velocity variation, and round-to-round
dispersion of the subcaliber spotting ammuni¬
tion. With the assumption of an “idea!”
spotter system (no trajectoiy mismatch) the
errors due to cant, range estimation, and
crosswind are eliminated. This assumption is
reflected primarily in zeroing error since the
errors due to cant and crosswind are
differential in nature if trajectory mismatch
does exist.
The total standard deviations for lateral
and vertical directions for a mounted
recoilless rifle with a spotting rifle become
MtotaU -(<*,,
+<**** + 4p) ,b < 7 - 9)
and
oJtotal) = (cr? + + 0 ? + <t5
V /S V UVU »nvs
+<ri +o* +*i y /i
V RRU *RXS *APJ
(7-10)
Table 7-1 also contains the magnitudes of
errors associated with recoilless rifles which
use spotting nfles.
3. Linearization of Error. As stated pre¬
viously, in order to breakdown the errors into
their components along the lateral and
vertical directions, it is necessary first to
convert all errors into linear errors For
example, some cf the linear standard
deviations of the errors in the vertical
direction an the target plane are computed as
follows (Ref. 2):
AMCP 70*238
TABLE 7-1
MAGNITUDE OF ERRORS FOR CALCULATING HIT PROBABILITY (Ref. 2)
Standard Dentation
Sours*
Symbol
Vertical k
Horizonte! x
Range estimation
°RE
21% /»* c
-
Round-to-round, main vmpon
a RRU
0.35 mil*
035 mil*
0.5 mil *
C5mlt*
Round-tor ound. spotting weapon
°RRS
05 mil*
05 mil 6
Muule velocity, main weapon
a UVM
lOtp** 6
Muzita velocity, spotting weapon
a uvs
lOfp* 6
—
Cant
°C
3 dag*
Wind
°w
-
11 fpa*
1 fp* 6
Aim point
a AP
1 .25 ft * 6
135 ft*- 6
Fire control
°FC
0.1 mil * 6
0.1 mil*- 6
Jump, main weapon
°JM
0.1 mil**
0.1 mil* 6
Jump, spotting weapon
°JS
0.1 mil 6
0.1 mil 6
Zaroing
round-to-round, main weapon
° zprm
°.mM “
<w/v5" **
round-to-round, spotting weapon
0 ZRRS
° fiHS *
a KRS b
jump, main weapon
°ZJU
0.1 mil*'*
0.1 mil* 6
jump, spotting weapon
0 ZJS
0.< mil*
0.1 mil 6
*M 18 , M 20 and M 27 Rifles, l*., r»Um having no ranging davict
*M 40 #nd M 67 Rifles, i.t., riflat having a ranging dwica
c /f - range
Oy RS = Or* tan w
a *iuui ~ aRH>l X R
°»RSS ~ ° RRS X R
_(*y\
“’MV* ~ Xw)**™
C *VS = ^v) 0>tYS
where
w * angle of fall, deg
R -■ range, ft
V - muzzle velocity, fps
7-3.3 LATERAL AND VERTICAL SINGLE
SHOT HIT PROBABILITIES
It has been stated earlier that the delivery
error can be characterized as being distributed
normally in two dimensions, lateral and
vertical. The single shot hit probabilities for
the two fire control systems are presented.
AMCP 708-238
1. Without Spotting Rifle. With the as¬
sumption of no fixed bias errors, in the case
of the weapon without a spotting rifle, this
establishes that the mean of the errors in the
lateral and vertical directions will be at the
center of the target (r = 0, y = 0). For the
recoilless weapon without spotting system,
the lateral p k and vertical p„ single shot hit
probabilities, assuming normal distribution of
errors, for a target that extends from - a to
+ a along both x- and y-axes are
dimensionless (7-12)
and
dimensionless (7-13)
2. With Spotting Rifle. In the case of the
tccoilless rifle with spotter system, the lateral
p h and vertical p y single shot hit probabilities,
again assuming a normal distribution of
errors, can be expressed as
(7 ' 15)
where
& h = mismatch between main and spotting
rifles in lateral (horizontal) direction,
ft
= mismatch between main and spotting
rifles in vertical direction, ft
The total single shot hit probability p H is
then the product of the separate single shot
hit probabilities in the laieral and vertical
directions, i.e., Eq. 7-6.
74 USE OF SPOTTING ROUND
74.1 GENERAL
As in all weapon systems, it is desirable to
obtain a high first round hit probability when
using recoilless weapon systems. In some of
the antitank recoilless rifles, the trajectories
are considerably arched and tend to reduce
the first round hit probability unless the
target range is known accurately. In order to
supply range information, many recoilless
rifles make use of a spotting system that
consists of a small caliber semiautomatic
spotting rifle rigidly attached to the major
weapon.
The purpose of the spotting rifle is to fire a
projectile containing a tracer in its base and
an incendiary or spotting mix in its nose so
that both the flight path and impact point are
visible to the gunner. In operation, the gunner
fires the spotting rifle until a hit on the target
is obtained, whereupon the major weapon is
fired. If the major projectile follows the same
path as the spotting rifle, the target will be hit
provided there is small disperson with respect
to target size. Ideally, the major weapon and
spotting systems should be designed to obtain
the same trajectory. The process of achieving
this correspondence is known as matching.
The difference between trajectory heights or
horizontal positions is defined as mismatch.
One of the basic matching problems is
caused by the difference in the exterior
ballistics of the major and spotting rifle
ammunition. Since the sm*li spotting projec¬
tile decelerates more rapidly, it will fall below
the major projectile trajectory if both
projectiles are given the same muzzle velocity.
In order to compensate for the difference in
caliber, the spotting projectile often is fired
with a higher muzzle velocity and at an
elevation slightly less than the major weapon.
The difference in elevation is built into the
spotting rifle mount and fc called bias. While
AMCP 706-238
somewhat effective for short ranges, this type fin-stabilized projectiles. Fin-stabilized major
of fix becomes marginal for larger ranges and projectiles with a slow right hand spin have a
inadequate in the presence of moderate slightly nose-up attitude to the left of the
crosswinds. projectile path, whereas most spin-stabilized
spotting projectiles have a nose-up attitude to
One method that helps the smaller spotting the nght of the projectile path. In order to
projectile better maintain its original velocity, compensate for the aerodynamic forces that
so as to make its flight velocities and times cause the spotting projectile to curve to the
more equal to the major projectile, is to right, it is necessary to introduce a small bias
increase the ballistic coefficient of the angle in azimuth between the major and
spotting projectile. The ballistic coefficient of spotting rifles,
the spotting projectile can be increased in
four ways: (1) better streamlining, (2) the use 7-4.2 MAGNITUDE OF MISMATCH
of higher density materials, (3) longer
projectiles, and (4) larger caliber spotting Tables 7-2, 7-3, and 7-4 contain the
projectiles. calculated matching velocities, vertical and
lateral bias angles, and the residual vertical
The remaining matching problems are and lateral mismatches for three major
associated with recoilless rifles that employ recoilless weapons with spotting rifle systems.
TABLE 7-2
MAGNITUDE OF MISMATCH SYSTEM 1 (Ruf.3)
Velocity,
Bias Avgfli, mil
AnMnunitiofl
fps
Vertical
Homtontd
105 mm, HEAT. T119E11
1650
Cal .50 ST.T189E1
1723
- 0.48
10.33
106 mm. HEP. T139E44
1635
105 mm. HE, T268
1606
Mismatch in Inch**
TH9E11 T189E1 T189E1
Minus T188E1 Minus T138E44 Minus 1288
Ranp.yd
V
H
V
H
V
H
400
0.4
3.3
1.0
- 7.9
4.5
- 8.0
500
-0.4
3.6
600
-0.8
3.5
3.5
-14.8
10.6
-14.9
700
-0.9
3.3
800
-0.4
2.5
4.4
-24.1
12.7
-24.2
900
0.7
1.4
1090
1.9
0.1
1.4
-36.9
6.5
-363
1100
1.2
-2.0
1200
-?.1
-4.5
-4.7
-52.6
-13.0
-51.4
7-10
AMCP 70*238
TAftILE 7-3
MAOiMTUOE OF MISMATCH SYSTEM 2 (!U*.3)
vdocRyi
Vwtfesl
Bias A«0a, mil
HoriMcSM
106 mm. HEAT. Til 8E11
1660
Cal .50ST. T18BE1
1816
-137
1030
106 mm. HEP. T130F 1
1601
1O6mm.HE.T208
1660
RAmA i* Inch—
TT19E11 T1S0E1 T1S0E1
Mteus T11Sei Mbw»T139644 MMytTMS
9Mft*,y4 V H V H VI
400
-4.0
ao
-6.0
- 73
-3.7
- 7.4
600
-2.2
ai
600
-0.4
a2
-63
-13.7
2.0
-13.9
700
1.4
Z9
800
Z6
2.3
-2.1
-22.8
63
-22.7
900
2.4
1.1
1000
1.1
ao
1.0
-343
5.4
-34.1
1100
0.6
-1.6
1200
-2.0
-4.1
2.8
-49.4
-5.2
-48.3
7-6 PROBABILITY OF HIT WITH RECOIL- first group consists of first generation
LESS RIFLES recoilless weapons that do not have a spotting
device. This group of recoilless rifles firing
7-6.1 COMPARISON OF SIMPLE SIGHT spin-stabilized ammunition follows:
AND SPOTTING ROUNDS
Rifle
Ammunition
Fig. No.
Fig. 7-1 shows the hit probability vs range
of a typical recoilless rifle with and without a
57 mm M18
M306A1 HE
7-2
spotting system. This figure definitely indi¬
cates the advantage of using a spotting rift''
57 mm M18
Projectile
M307 HEAT
7-3
beyond the range of 400 yd.
75 mm M20
Projectile
M309 HE
7-4
7-6.2 PROBABILITY OF HIT FOR STAND¬
ARD WEAPONS
75 nun M20
Projectile
M310 HEAT
7-5
Figs. 7-2 through 7-9 show the results of
105 mm M27
Projectile
M323 HE
7-6
first round hit probability computations for
the ret of error sources described in par. 7-3.2
105 mm M27
Projectile
M324 HEAT
7-7
as a function of range for eight standard
recoilless rifles which can be divided into two
groups according to their fire control. The
The second
Projectile
group of rifles
consists of
7-11
AttCP70ȣM
TABLE 74
MAGNITUDE OF MISMATCH SYSTEM 3 <IM. 31
Vriocity,
DiM Amis
mil
AMMHIflitiOfl
Vertical
Horizontal
106 mm. HEAT, T119E11
1650
Cal .50 ST.T189E1
1626
2.44
10.72
106 mm. HEP. T139E44
1596
106 mm, HE. T268
1522
Mto
matdi in Indias
T119E11
T18ME1
TttSEI
Kftmat T188E1
Minus T138E44
Minus T2SB
V H
V
H
V
H
400
-26.4 8.6
33.4
- 1X5
45.0
- 13.7
500
-29.0 11.1
600
-2a9 11.4
44.7
- 23.2
67.4
- 23.6
700
-25.5 12.2
800
-18.8 12.8
47.3
- 353
853
- 363
900
- 9.0 12.9
1000
3.4 12.5
38£
- 50.6
93.6
- 51.6
1100
16.8 11.6
1200
28.8 10.5
23.7
- 70.3
88.8
- 70.3
1300
33.9 8.6
1400
27.8 6.4
6.2
- 93.4
6d.6
- 92.4
1500
19.3 3.6
1600
9.0 0.0
- 8.9
-122.1
32.0
-118.1
1700
2.0 - 4.1
1800
- 9.1 - 8.6
-14.6
-156.4
- 22.3
-147.5
1900
-21.4 -13.9
2000
-30.7 -19.6
-11.5
-197.4
-100.7
-180.8
newer tecoilless rifles that tire spin-stabilized As shown in Figs. 7-2 through 7-9 the
ammunition and are equipped with a spotting weapon systems which employ spotting rifles
rifle, i.e..
Rifle
Ammunition
Fig. No.
have much higher first round hit probabilities
than those rifles which rely on visual range
estimation. Figs. 7-8 and 7-9 also show that
90 mm M67
M37I HEAT
7-8
the hit probabilities of those weapons with
spotting rifles remain high even up to their
106 mm M40
Projectile
M344 HEAT
7-9
maximum range.
Projectile
7-12
-,\V
a
i
i
<
1
j
.¥
Figure 7-7. Probability of Hit - 705 mm M27 Rifle;
M324 HEAT Projectile (Ref. 2)
TJ
C
o
a
a
a
o
H
fO
-Q
O
u.
0*
a
Range, ft
I
■<
\
Figure 7-8. Probability of Hit - 90 mm M67 Rifle;
M371 HEAT Projectile (Ref. 2)
AMCP 706-238
67 mm
75 mm
80 mm
100 mm
Weapon
57 mm
76 mm
90 mm
106 mm
TABLE 7-6
SINGLE-SHOT HIT PROBABILITY-VISUAL RANGE ESTIMATION (Reft. 5 AND 6)
Velocity, Vpt
fun**, yd
Single Shot Hit
Probablity
1200
200
0-99
400
0.37
600
0.08
800
0 02
1000
200
0.99
400
0-37
600
0.11
800
0.03
700
100
0.99
200
0.79
300
0.30
400
0.11
1600
200
0.99
400
9.62
600
0.24
800
0.12
1000
0.07
1200
0.04
1400
0.02
TABLE 7-6
SINGLE-SHOT HIT PROBABILITY-CRUDE RANGE FINDER (Raft. S AND 6)
Vaiodty.fpt
Rangt.yd
Singla Shot Hit
ProbabMHy p h .
1200
200
0.99
400
0.55
600
0.26
800
0.10
1000
200
0.99
400
0.57
600
0.28
800
0.11
700
100
0.99
200
0.81
30G
0.45
400
0.26
1600
200
0.99
400
0.65
600
0.39
800
0.23
1000
0.14
1200
0.09
1400
0.06
AMCf* 706-238
7-5 3 PROBABILITY OF HIT AS A FUNC- single shot hit probability, the total probabili-
TlOfJ OF VARIOUS CONDITIONS ty of at least one hit in two shots, and the
total probability of at least one hit in three
Tables 7-5 through 7-7 list typical single shots, respectively, as a function of muzzle
shot hit probabilities for several hypothetical ve: 'city. The ?2 figures are based on calcula-
recoilless rifle systems with either visual range tions for a medium sized recoilless rifle with
estimation, crude range finder, or spotting spotting rifle fired at a 7.5 ft by 7.5 ft vertical
rifle fire control systems. target. The independent and normally dis¬
tributed quasi-combat errors assumed to cause
7-F.4 PROBABILITY OF HIT AS A FUNC- impact deviations at the target are shown in
TION OF MUZZL C VELOCITY Table 7-8. The quasi-combat errors listed in
Table 7-8 are the errors expected in a typical
Figs. 7-10 through 7-12 show the total combat situation.
AMCP70C-238
Range
Figure 7-9. Prabcbitity of Hit — 106 mm M40 Rifle;
M344 HEAT Projectile (Ref. 2)
TABLE 7-8
INDEPENDENT AMD NORMALLY DISTRIBUTED QUASI-COMBAT ERRORS
ASSUMED TO CAUSE IMPACT ERRORS (Rtf. 7)
Standard Deviation
Cause
Vertical
Horizom
Round-to und, Main Weapon
0.5 mil
0.5 mil
Round-to*Round, Spotting Weapon
0.5 mil
0.5 mil
Jump, Main Weapon
0.25 mil
0.25 mil
Jump, Spotting Weapon
0.25 mil
0.25 mil
Muzzle Velocity, Main Weaponto-Weapon
7.5 fps
-
Muzzle Velocity, Spotting Weapon-to-Weapon
7.5 fps
-
Aim Point
1.25 ft
1.25 ft
Crosswind
-
11 fps
Cant
—
40 mils
7-19
hot Hie Probability
Total Probat
Spotting Rifle
Quasi-combat Errors
7.5' x 7.5* Target
Range^ yd
Figure 7* 11. Effect of Muzzle Velocity on Probability of One Hit Out of Two Shots (Ref. 7)
Probability of At Least One Hit In Three Shots
Figure 7-12. Effect of Muzile Velocity on Probability of One Hit Out of Three Shots (Ref. 7)
AMCP 706-238
SFdnMIli
KILL mOBABILITY
?*§ tNYAODUCTIQM
Kill p^^Hbility p K is defined as the
product (.** total hit probability p u and
conditional probability of a kill given a hit
Pk\h' where kill means to destroy the target
to the degree defined in par. 7-7.2. From this
general definition it is seen that, given a kit on
a target, ike calculation of kill probability
depends only on the type of warhead used
against a specific type of target. In this
general sense, kill probability is a measure ot
the effectiveness of the weapon system in that
it involves the delivery accuracy of the
weapon system and the lethality of the
warhead on a partie'lar target.
7-7 HARD TARGET
7-7.1 INTRODUCTION
Kill probability is dependent upon the
particular .ype of target. The first type of
target to be considered is the hard target that
is typified by the type of target a tank
presents. The hard target is small in
dimensions in contr, t to an area target over
which personnel are scattered and, in general,
requires a hit on the target in order for a kill
to occur. Most hard targets arc protected by
some type of armor and generally will require
penetration of tile armor by a HEAT or HEP
type warhead in order to defeat the target.
7 7.2 T7PES0F KILL
The three categories or types of kill of an
armored target are defined (Ref. 8):
1. K Kill, 't't’c lype of kill in which the
armored target is destroyed and is dependent
to a great extent on the ignition of fuel or
ammunition.
2. F Kill. The type of kill which causes
complete or partial loss of the ability of the
tank to fire its main armament and machine
guns.
3. M Kill. The type of kill which causes
immobilization of the tank.
As detailed in Refs. 8 and 9, a list of
standard damage assessments can be estab¬
lished to be able to quantitatively define
which type of kill has taken place.
7-7.3 VULNERABLE AREA
The vulnerable area of a target is defined as
the product of the presented area of the
target and the probability that a hit on the
presented area will be a kill. Since the
presented area of a particular target is known,
the evaluation of the vulnerable area depends
upon determining a value for the probability
that a hit on the presented area will be a kill.
Considering two types of warheads, HEaT
and HEP, which could be used in attempting
to defeat a hard target such as a ♦ink, one can
see that this probability function will depend
on where a specific type of warhead strikes
the target or if the warhead penetrates the
armor. It will be necessary to know the
probability of the warhead penetrating the
armor at 'die specific point of the target and,
if penetration occurs, the probability that the
warhead will cause tile defeat of essential
components of the target, in certain areas of
the tank, it will only be necessary for the
7-23
A%KF7G*£a»
projectile to penetrate tlie armor in order to
cause a kill, whereas hits on or near
areas-such as the suspension and turret
ring-may be enough to cause an M-typc kill
of the target.
Vulnerable areas arc calculated separately
for each of the three types of kill described in
par. 7-7.2 The general method of calculating
the vulnerable area of hard targets for HEAT
and HEP type warheads is given in the next
subparagraph.
7-7.4 CALCULATIONS OF KILL PROBA¬
BILITY
Kill probability with a single shot as earlier
defined can be written in the following
manner:
Pk~PhPk\h (7-16)
where
p x = kill probability
Pu = total hit probability
Pk\h* conditional probability of a kill
given a hit
The hit probability of any point on the
target is determined by the methods described
in Section II so that upon determining the
value of Pk/h, the kill probability essentially
is determined. The definition of vulnerable
area A „ is
■A* = Pk\ h (7-17)
where
A v - vulnerable area, in?
A = presented area, in?
With Eq. 7-17, the kill probability can be
expressed a' the product of the hit probabil¬
ity and the ratio of vulnerable area to
presented area or
Px = Pa (dv.) (7-18)
\A/
More precisely, the vulnerable area for a two
dimension target is given by the following
relationship (Ref. 1):
A„ =
jj^Prix,y)dxdy
(7-19)
where
x,y
= coordinates centered at the cen¬
ter of gravity of the target and in
a plane perpendicular to the
projectile trajectory
Px( x >y )~ probability of kill function for a
hit at the point (x, y)
The probability-of-kill function p x (x, y)
assigns a probability value that the target will
be killed if there is a hit at tlie point ( x, y).
A complex target such as a tank can be
considered to be made up of individual
vulnerable components. If the components do
not overlap or mask one another and are no.
redundant, the total vulnerable area is merely
the sum of the vulnerable areas of the .2
components.
In practice, vulnerability drawings of the
target are prepared, showing the arrangement
of the interior components to the line of fire.
By use of an overlay grid, a conditional kill
probability is entered into each square of the
grid for a given point of aim. This probability
value depends upon the damage that would
result on the component behind the particular
grid square as a result of a specific type of
warhead striking the designated point (*, y)
of the target and is determined from target
7-24
AMCP 70S-23C
vulnerability studies and existing vulnerability
data on similar components. The vulnerable
area of the whole target for one of Lie three
types of kill then would be computed by
summation of the component vulnerable
areas, taking into account masking and
redundancy.
7-7.5 r/PICAL VALUES OF KILL PROBm
BILITY
Fig. 7*13 obtained from Ref. 7 shows the
expected variation of kill probability with
range, caliber, and muzzle velocity for several
hypothetical recoilless weapon systems when
fired against a JS III tank. The kill potential
of these weapons was made on the basis of
the quasi-combat error conditions as defined
in par. 7-S.4. As shown in Fig. 7-13, the
combined effect of caliber and muzzle
velocity on the expected number of kills is
dearly evident. As expected, the longer the
range, the lower the kill probability and, for
any selected value of range and muzzle
velocity, the larger the caliber of the weapon,
the higher its kill probability.
7-8 AREA TARGET
7-8.1 INTRODUCTION
The recoilless rifle weapon system also is
employed in the defeat of personnel distrib¬
uted over an area. This type of target is called
an area target and is a definite contrast to the
hard target described earlier. The area target is
usually large in dimensions and, except for
foxholes and cover by the natural terrain,
there is no armored protection for the
personnel within the area. The defeat of
personnel in the area target is accomplished
by the fragmentation of the HE type or
antipersonnel type warheads.
7-8.2 LETHAL AREA
Evaluating the lethal area of a fragmenting
projectile permits the prediction of how many
casualties a projectile will produce upon
detonation under specified conditions. The
basic concept used in arriving at a suitable
lethality index is that of the expected number
of incapacitations E(K) of personnel targets
which is given by
E00 = jj^oix,y)Pxix,y)dxdy (7-20)
where a(x, y) is the density of the personnel
targets and p K is the probability of a target
within area A being incapacitated from a hit
at a point ( x, y ). For a constant target density
Eq. 7-2G becomes
E(K) = oix,y)Jj^Pxdxdy (7-21)
Lethal area L A is
L x = \\ A Pxdxdy (7-22)
The probability p K is really the joint
probability of the target being exposed, the
target being hit if exposed, and the target
being incapacitated if hit. The valuation of
the function p K depends upon the following
parameters:
1. Projectile angle of fall
2. Projectile terminal velocity
3. Burst height
4. Projectile external geometry
5. Projectile internal geometry
6. Projectile casing composition
7. Projectile filler composition
8. Target presented area, which depends
on:
a. target attitude
b. cover (natural or artificial)
7-25
AMCP 706-238
c. fragment aspect angle
9. Target incapacitation critera.
As seen from this list, the function p K , and as
a result L A , i* only a function of the warhead
and target area characteristics and is indepen¬
dent <\* \:apon accuracy.
REFERENCE -
1. AMCP 706-327, Engineering Design Hand¬
book. Fire Control Series, Section 1, F/re
Control Systems - General .
2. David E. Walters and Edith F. Redly,
Hitting Probabilities of the Standard Re¬
coilless Weapons . Memorandum Report
M59-32-1, Frankford Arsenal, Research
and Development Group, Philadelphia, Pa.,
June 1959.
3. AD 34245, Symposium on Recent Prog¬
ress of Recoilless Rifles and Ammunition ,
Department of Army, January 1954.
4. Recoilless Rifle Handbook (Unpublished),
prepared at Frankford Arsenal, Philadel¬
phia, Pa.
5. AD 392 365, Frank A. Lepoid, Jr. and
Ned I. Yates, Jr., An Effectiveness Study
of Infantry Antitank Weapons, Technical
Memorandum No. 13, Aberdeen Proving
Ground, Maryland, August 1968.
6. AD 351 905, Capt. L. R. Creelman, A
Parametric Study of the Probability of Hit
of Unguided Ballistic Weapons, Canadian
Armament Research and Development Es¬
tablishment, Valcartier. Quebec, March
1965.
7. MAW Long Range Report 1303-20 (Un¬
published), Frankford Arsenal, Phila¬
delphia, Pa.
8. AMCP 706-245(0 Engineering Design
Handbook. Artillery Ammunition Series,
Section 2, Design for Terminal Effects (U).
9. AMCP 706-170(S), Engineering Design
Handbook, Armor and Its Applications
(U).
BIBLIOGRAPHY
AD 119-560, Estimates of a Spotting System
for the T41 Light Tank, Pitman-Dunn Labora¬
tories Group, Frankford Arsenal, Memo¬
randum Report MR-637, December 1956.
An Analytical Comparison of Fin and Spin-
Stabilized Spotting Projectiles for the Ulti¬
mate Battalion Antitank Weapon System ,
Phase Report No, 1, Midwest Research Insti¬
tute, Contract No. DA-23-072-ORD-901, for
Frankford Arsenal, Philadelphia, Pa., 15 Au¬
gust 1955.
AD 312-558, Accuracy of the Recoilless Light
Assault Weapon,
AD 3826, Helen J. Coon and Frank E.
Grubbs, bKL Memorandum Report No. 636.
On Estimating Probabilities of Hitting for the
Battalion Antitank Weapon, January 1953.
AMCP 706-107, Engineering Design Hand¬
book, Elements of Armament Engineering,
Part Two, Ballistics .
David Walters, A Spotting Rifle for the 90
mm Gun Mounted on the T42 Tank, Report
R-1123, Pitman-Dunn Laboratories, Frank-
ford Arsenal Philadelphia, Pa., April 1953.
J. L. Wright, Dynamic Tests for the Light
Measurement on Caliber ,50 Spotter-Tracer
Ammunition, Pyrotechnics Lab. Report
AMCP 706-238
Picatinny Arsenal, Dover, N.J., September
1961.
R. T. Eckenrode, The Spotting Technique.
Memorandum for Record, Project TS4-402f!
Frankford Arsenal, Philadelphia, Pa., Nove n.
ber 1954.
E. Arnold, Sensitivity Test for Primer Compo¬
sitions. PIC Bulletin No. 10, Frankford Arse¬
nal, Philadelphia, Pa., June 1942.
AD 153-036, Harold Brodkin, Fire Control
Siudics-Tank Gunnery Accuracy Evalmtion.
Report R-1380A, Frankford Arsenal, Phila¬
delphia, Pa., February 1958.
D. E. Walters, Hit Probability of T114 BAT
Vehicle System, Memorandum Report
M63-8-1, Frankford Arsenal, Philadelphia,
Pa., August 1962.
D. E. Walters and E. F. Reilly, Hitting
Frequency of the 57 mm T66 Recoilless
Rifle, Technical Memorandum No. M61-5-1,
Frankford Arsenal, Philadelphia, Pa., Septem¬
ber 1960.
7-28
AMCP 706-238
CHAPTER 8
MEASUREMENT TECHNIQUES
8-0 LIST OF SYMBOLS
A = bore area, in?
A i = piston area, in?
a g - sonic speed, fps
C - unit of electrical ciiarge, C
C L ® line capacitance, pF
C T = total input capacitance, pF
c - speed of light, fps
D = horizontal displacement of per-
dulum, in.
D [ u,ru 4 = spacing between fixed points for
velocity measurement, ft
F = unit of capacitance, F
f d = Doppler frequency, Hz
f R = radar operating frequency, Hz
g = acceleration due to gravity, ft-
sec' 2
GF - gage factor, dimensionless
I = current, A
/ = recoil impulse, lb-sec
K = gage constant, Ib-OiCF 1
N - number of cycles of the Doppler
aial measured from time equal
zero, dimensionless
P = internal chamber pressure, psi
P - partial pressure of air at tempera¬
ture T, psi
P b * peak blast pressure, psi
P„ ~ pressure constant, psi-V 1
P = atmospheric pressure, psi
P - partial pressure of water vapor
temperature T, psi
R ~ resistance, ohm
R g ~ gage resistance, ohm
R r = retardation of projectile velocity,
fps-fr*
S v ~ output voltage charge, pvolt
T - air temperature, °F
t = time, sec or psec
/,, t 2 - time to traverse fixed distances
for velocity measurement, sec
V = power supply voltage, V
V m = muzzle velocity, fps
8-1
AVCP 706-238
V p = projectile velocity, fps
Vx * radial velocity, fps
V ( - velocity of shock wave, fps
V T = triggering voltage, V
Vi, V 2 = average projectile velocity ob¬
tained from time to traverse fixed
distances, fps
W = projectile weight, lb
W t = weight of pendulum and recoil¬
less rifle, lb
x = displacement, ft r
1
- acceleration, g’s v
= ratio of specific heats of air = 1.4,
dimensionless
= strain, pin.-(in.) _1
= angle of radar beam with respect
to projectile trajectory, deg
- radar wavelength, ft
= 10" 6 , dimensionless
= period of pendulum, sec
X
7
€
0
X
8-2
iwv-aws! i j i ( imuintg »mnK U ’iw mm jwwA vJaeww« ■. .vv^niwwan,
AMCP 706-238
)
SECTION I
'NTRODUCTION
The experimental studies performed in the
design of recoilless systems require the use of
a variety of special purpose instrumentation
to record important test data. Determination
of flight characteristics may require measure¬
ment of velocity at a number of points along
the trajectory as well as at the muzzle,
determination of yaw, spin rate, and projec¬
tile integrity in flight. Interior ballistic
performance studies will require, at the least,
information on pressure-time history in the
chamber, muzzle velocity, and may, in
addition, require information on interior
transient temperatures, projectile displace¬
ment-time, and projectile base pressure
measurement. Nozzle throat design will
require information on recoil impulse and
possibly recoil force and velocity-time his¬
tory. Investigation of firing safety hazards
may require a plot of the blast pressure field
around the system and additional information
such as muzzle flash and recoil torque also
may be necessary. Design of the gun tube
often will require experimental verifi tion of
stress levels at various points in the weapon
and temperature measurements during rate-
of-fire tests.
The short duration of the transients
t associated with a system of this nature-
1 coupled with the high pressures and tempera¬
tures, and the difficulty of ready attachment
of measuring devices to a lightweight
system—may make the use of especially
designed instrumentation, recording equip¬
ment, and test weapons necessary. Recording
equipment used for tests of rocket devices
and other slow varying phenomena usually is
not suitable.
While most equipment components neces¬
sary for the recording of recoilless rifle
experimental results are now available com¬
mercially, proper selection and application
will be described in this chapter and
construction of certain special purpose
devices explained. It will be assumed that the
reader has a familiarity with basic principles
of measurements and general equipment
available. The systems mentioned have been
tried and ore practical; however, the rapid
advance in the technology of measurements
may in some cases have already obsoleted
them in favor of more reliable or accurate
equipment. Use of such equipment is
recommended where one can ascertain that
equivalent or better performance is possible,
however, thoughtful consideration should be
given to the dynamic environment encoun¬
tered in the system and assurance made that
the specification on the equipment chosen
will apply in such an environment.
1
1
8-3
i
AMCP 70*338
SECTION II
MEASUREMENT OF VELOCITY
8-1 GENERAL
Probably the most important measurement
in the design of the overall system is that of
velocity of the projectile. It is desirable to
know the velocity of the projectile at all times
from ignition of the propellant to target
impact, however, the most often used
measurement is that of velocity at the muzzle
of the gun.
Muzzle velocity usually is determined from
the tune taken for the projectile to travel
between two detectors a known distance
apart. This time is measured with an
electronic time interval meter or chronograph.
Since this method gives the average velocity
over the distance of measurement, it is
necessary to record two ir more velocities
ahead of the muzzle and extrapolate back to
muzzle velocity. A diagram of a typical setup
is shewn in Fig. 8*1. From the arrangement of
Fig. 8-1, the following may be determined:
Average velocity: Vj = D 2 /t x , fps \
Average velocity: V 2 = D x /t 2 , fps /
Retardation R r :
~2 +D S + ~2
fps-(ft)- 1
Considering the retardation to be linear,
which is approximately true for short
distances (e."g., 100 ft), the muzzle velocity
V m is given by the equation
V m = V x +Z> 2 /2) (8-2)
When range space is limited, it may be
desirable to stagger detectors as shown in Fig.
8-2 to achieve greater velocity accuracy by
permitting longer baselines with a reduction,
however, in distance between velocities and
hence an increased retardation error. From
tl,e arrangement of Fig. 8-2, the following
may be determined:
V x = (1% + D 9 )/t x \
V 2 = U) t +D i )/t 2 I
> ( 8 - 8 )
R T = 2(V 1 -V 2 )/(J) 2 +D i ) l
V m = V x +R,[D X + (D 2 + Z> 3 )/2 ] /
Errors inherent in both these methods are:
1. Error in distance measurement (usually
in the range of ±0.01 ft)
2. Detector error-error caused by time
delays in the detector uncertainty in projec¬
tile location at which the electrical pulse
output is generated
3. Error in time measurement-±1 psec
with a 1 MHz time interval meter, providing
that electrical time delays do not occur in
transmission lines between detectors and
meter.
The use of a long baseline (distance
between detectors) can decrease both timing
and distance measurement errors; however, its
length often will be limited by firing range
facilities, especially if a rather high firing
angle is used. It is necessary to make two
velocity measurements for accurate deter¬
mination of muzzle velocity; if a linear
extrapolation is used, the two velocity
measuring systems should be close together.
The determination of muzzle velocity, as
Preceding page blank
n'lft ti ttii 11 i r lit!
AMCP 706-233
Figure 8 - /. Velocity Measurement Scheme >ic
described, actually gives the velocity at some
point slightly forward of the muzzle since
escaping gases accelerate the projectile after it
leaves the muzzle. In normal recoilless
systems this increase is not significant due to
the reasonably low pressures and velocities
encountered. It is desirable to locate the first
velocity detector some distance from the
muzzle to prevent muzzle blast or flash from
affecting the detector performance. This
distance may vary from 15 ft for a 57 mm
gun to as much as 50 ft for high velocity
larger caliber guns. The recommended base¬
line for a velocity system is in the nature of
25 to 50 ft to assure an error of ±0.1 percent
(± 1 fps at 1000 fps) or less in velocity caused
by distance measurement and detection error.
8-2 DETECTING DEVICES
The purpose of the detecting device is to
produce an electrical signal indicating the
passage of the projectile at a known point in
space. There are several types of detectors
>—I l
_r
Figure 8-2. Velocity Measurement With Staggered Array of Detectors
AMCP 706-238
suitable for velocity determinations of recoil¬
less systems, each of which have certain
advantages and disadvan tages.
8-2.1 8REAKW1RE SYSTEM
'Pus system consists of a grid of wire, or
paper with a conductive grid strung across a
frame made of an insulating material such as
wood. The wire is broken by the passage of a
projectile through it. Normally, a current is
passed through the wire, and a chronograph is
used to sense the reduction in current when
the wire is broken. While this is probably the
most simple detecting device, it requires
replacement of the wire after each firing. In
addition, the wire has a tendency to stretch
before breaking, especially when pointed
projectiles are used, causing an error in
baseline measurement. This error may be
minimized by using hard drawn wire, keeping
it stretched taut, and using narrowly spaced
g rid wires. The circuit used mu> iliL system is
shown in Fig. 8-3.
The breakage of the wire will cause the
voltage across the terminals of !ie chrono¬
graph or time interval meter to rise to V,
considering that the input resistance of the
counter is high with respec* t<> R. Since .he
line between the breakwire and the chrono¬
graph normally will have a capacitance C L ,
the rise to V will not be instantaneous. The
approximate value for any value of C L the
line capacitance, and V T the triggering voltage
of the chronograph may be determined from
the equation
/ = V/R = C L V T /t, A (8-4)
(for V T < V, C L <C AC )
where
/ = current in closed circuit, A
V - power supply voltage, volt, >50 V T
R - series resistance, ohm
Q= capacitance of line, chronograph
input and break circuit, pF
t - signal delay time, psec, for wor
permitted
V T - triggering voltage of chronograph
example, V
An example of the application of Eq 8-4
follows:
8-7
AMCP 70*238
Given:
8-2.2 MAKE SYSTEM
t * 1 psec
C L * 0.005 mF
V T » 1 volt
V - 50 volts
The use of IC* c is to provide AC coupling
into the chronograph if required. This
example considered the value of the break-
wire resistance to be low in comparison to P.
Since the length of wire for the two breakwire
circuits usually will be similar, the signal
delays will be similar and the timing error will
be less than that caused by either one alone.
Where long cables or higher triggering
voltages are required, it is desireable to use
pulse-shaping and line-matching techniques
which are well known in electronics, see Fig.
8-4. For example, one might have an
integrated-circuit connected as a Schmitt
trigger feeding into a 50-ohm cable. If the
input and output impedances are appropri¬
ately matched, the pulse will suffer little
degradation in shape (or amplitude) even over
fairly long distances. Electronic technicians or
equipment vendors ought to be consulted for
the proper equipment to use. Circuit delays
can be made the same by making the 2 cable
lengths the same, or the equipment can be set
to account for the different delays automat¬
ically.
The make-circuit consists in principle of
twc conductors, separated by an insulator,
which are connected by the passage of the
projectile. In practice, the system consists of a
sandwich of sheets of aluminum foil glued on
Styrofoam about one inch thick. Screen wire
electrodes with hardboard or thin plywood as
a separator also have been used successfully.
The make circuit is especially useful in the
measurement of terminal velocity where large
size screens are necessary and replacement of
break wires difficult. Make screens as large as
20 feet square have been fabricated and used
for many firings before being destroyed to the
point where some contact is not made
between the projectile and the two electrodes.
Other materials have been tried including a
sandwich of foil and cardboard; however, it
was found that the insulator tended to
extrude over the rear foil and prevent contact.
When using a separator other than Styrofoam,
it is advisable to leave an air space between
the rear electrode and the insulator to provide
good contacts. The circuit, Fig. 8-5, used is
similar to that used with the break circuit.
The circuit has a time delay proportional to
both the series resistance of the line and
battery, and the capacitance of the line. For
V large with respect to the triggering voltage
and the line resistance R, small with respect
to chronograph input resistance, the approxi¬
mate time required t to reach the triggering
voltage V T will be
t = C L VjJt/V, Msec (8-6)
where
t - time, psec
C L - capacitance of line plus make
circuit, mF
V T - trigger voltage, V
V = supply voltage, volt
R = series resistance of battery and line,
ohm
FIRST SIGNAL PATH
AMCP 706*238
Tim*-d*Uy
Manuring
•nd Speed
Computing
Equipment
SECOND SIGNAL PATH
Figure 8-4.
Measuring Projectile Speed
An example of the application of Eq.
8*5 Determine: t
follow*:
Given:
0.01 x l x 100 i
,= 45 -45 '““
Cl
* 0.01 itF
V T
■ 1 volt
(Negligible compared to other circuit delays).
V
* 45 volts
For long lines or where a high trigger voltage
is required, refer to the paragraph just above
R
* 100 ohm
par. 8*2.2.
1 1
1
COUNTER I 1 I
Figure *5. Make System Circuit
AMCP 706-238
8*2.3 SOLENOID COIL DETECTORS
Probably the most used method for the
velocity detection of recoilless rifle projectiles
is the solenoid coil system where a magne¬
tized projectile passes through a coil of wire
to produce a current, indicating its passage.
The coil normally is wound about 200 turns
of No. 20 to No. 24 magnet wire in a loop 20
to 30 in. in diameter, dependent on the
diameter of the projectile. While originally
wound loosely on a wooden frame, it was
found that the excessive blast from a
recoilless system caused enough vibration of
the wire in the magnetic field of the earth to
produce extraneous signals. Latter coils were
tightly bound and rigidly mounted to a
wooden donut-shaped disc.
It is necessary to magnetize the projectile
in the proper direction prior to firing, or, if
the projectile is nonmagnetic, to insert a
magnet where it will not be excessively
shielded by the material of the projectile. It is
common practice to mount a cylindrical
magnet in the nose so that at least 0.5 in.
protrudes beyond the projectile nose.
The design of the solenoid coil and the
pulse shaping circuit is important to assure
triggering of the chronograph at a kr.own
point in space. As shown in Fig. 8-6, the wave
shape of the signal from the coil is much like
a sine wave. As the projectile approacnes the
coil, the increasing magnetic flux induces an
EMF which reaches a maximum and then
rapidly drops to zero as the projectile field is
centered in the coil. The EMF then rapidly
drops to some negative value and sic ,;y
returns to zero as the projectile passes out of
the coil. It is advisable to use a shaping circuit
to pick off the point of rapid negative rate of
change, where the signal passes through zero,
as the trigger point for the chronograph. A
shaping adapter is available to perform this
function, or it may be performed on most
universal time interval meters having high gain
amplifiers by setting slope control to negative
and amplitude control slightly negative. To
ascertain the accuracy of trigger point, it is
useful during initial system test to record
concurrently on an oscilloscope the output
signal from the coil and the gate pulse from
the chronograph. Normally, coils are wired in
series and connected to a common (“period”
or “coils”) input on the chronograph. A
sketch is shown in Fig. 8-7.
Proper polarity of coils and projectile
magnetization may be checked by the use of a
compass and a DC polarizing current of about
100 mA applied across the coils at the
chronograph input. A rule of thumb pre¬
scribes that magnetization of the projectile
8-10
AMCP 706-238
Figure 8-7. Series Wiring of Coils
should be strong enough to deflect a compass
45 deg from the magnetic Held of the earth
when 4 in. from the nose of the projectile.
This test also may be used afh-r magnetization
to assure relative consistency of magnetic
field strength between projectiles. Projectiles
normally are magnetized by placement in a
coil equal to the length of the mqjor portion
of the projectile through which a steady or
impulsive high current is applied.
8-2.4 SKY SCREEN
Another method of detecting passage of a
projectile in space is the sky screen. It has the
advantages over other systems of not
interfering in the visual path of the projectile,
and permitting a number of velocities to be
taken down range. The device consists
basically of an optical system, collimating slit,
and photomultiplier tube which produces a
pulse when a rapid change in ambient light
level^ occurs in its field of view. It has a
fan-shaped field of view which will produce
an error of about 0.2 to 0.5 percent,
depending on the setup method, due to its
spread. For muzzle velocity, it is normally
positioned directly below the trajectory;
while for time of flight measurement, it is
placed off to one side to increase the field of
view. It cannot be used on dark or hnzy days
and and cannot be pointed into the sun.
Extreme care must be taken in positioning the
unit since a small change in angle of the lens
can cause a considerable error in baselines.
One method to determine the sighting point
of the device on flat trajectory firings is to
suspend objects at points directly above the
screens (when the screens are pointed
vertically) on the trajectory. A meter
measuring photomultiplier cathode current
will dip when the screen is pointed directly at
the object. Measuring the distance between
objects will give the baseline.
8-2.5 RADAR VELOCITY MEASURE¬
MENTS
Microwave interference (Doppler radar)
techniques may be used to measure the
velocity and displacement of the projectile in
the barrel and its velocity over its entire
trajectory. The basic system consists of a
microwave transmitter that transmits a signal
of known frequency in a beam along the axis
of the projectile and a receiver that receives a
signal reflected from the projectile. Trans¬
mitted and received frequencies are com¬
pared, and the difference or Doppler fre¬
quency obtained is proportional to the
projectile velocity along the axis of the
microwave beam by the relationship of Eq.
8 - 6 .
where
V R = radial velocity, fps
X = radar wavelength, ft
f d = Doppler frequency, Hz
8-11
AMO* 706-238
x = c /4 , ft
where
c ' speed of light, fps
4 = radar operating frequency, Hz
Since it is difficult to have the angle between
the radar beam axis and the projectile trajec¬
tory equal to zero at all times, the radial
velocity measured by the radar, i.e., the
component of velocity of the projectile in the
direction of the radar beam, will be somewhat
less than actual projectile velocity along the
axis of its trajectory. A typical setup is shown
in Fig. 8-8. The velocity V R measured by the
radar will at any point in space equal the
actual projectile velocity V p multiplied by the
cosine of the angle 0, i.e., V R = V p cos 0. In
recoilless firing experiments, it obviously is
not possible to locate the radar directly
behind the gun. It, therefore, is necessary to
locate the radar as close to the side of the gun
as possible, considering blast effects on the
equipment, to obtain good down range
measurements. For accurate muzzle velocities
or velocity of the projectile while in the
barrel, the radar may be located just off the
trajectory down range and pointed toward the
gun.
The radar systems produce a sinusoidally
varying signal proportional to the radial
velocity of the projectile which may be
recorded by a number of methods. For
recording the velocity over the complete
trajectory, the Doppler signal may be
converted by a frequency meter to a voltage
proportional to frequency and, hence, velo¬
city that may be recorded on an optical
oscillograph as a trace of velocity versus time
of flight. The signal also may be recorded
digitally as a series of points containing the
number of cycles of the Doppler signal
occurring in given time increments, i.e., a
number of velocities measured during the
flight of the projectile.
The microwave system may be pointed
toward the gun to measure velocity and
displacement within the bore as shown in Fig.
8-9. To reduce the effect of radar off the axis
of the projectile, the setup shown in Fig. 8-10
has been applied to advantage. Here a
reflector made of foil backed with Styrofoam
is placed about 25 ft forward of the muzzle at
an angle to reflect the signal into the gun. The
system is best aligned by placing a microwave
detector connected to a meter in the gun tube
and positioning for best signal strength as
indicated on the me w. Since, by Eq. 8-6
it also holds that
where
x = displacement of the projectile from
rest, ft
N = number of cycles of the Doppler signal
measured from time t = 0
By recording the raw Doppler signal on an
oscillograph, drum camera, or similar device,
it is possible to obtain projectile displace¬
ment-time information.
In recording Doppler information in a gun
tube, an error is caused by an apparent change
in the wavelength of the microwave signal
when traveling in a waveguide, which the gun
tube may be considered. The error is
significant when the diameter of the gun
barrel approaches the wavelength of the radar
system which is the case when the X-band
microwave equipment is used with normal
size recoilless systems. To avoid this effect,
the system may be calibrated statically by
positioning a piston in the gun at measured
amounts equal to null in the microwave
8-12
AMCP7M-23C
Figure 8-8. Radar Velocity Measurement Schematic
signal. In-bore microwave systems are de¬
scribed more fully in Refs. 2, 3, and 4.
The simplest radar system applied in
recoilless tests is shown schematically in Fig.
8-11. It consists of a highly stable X-band low
power klystron coupled through a “magic
tee” to a 10-in. parabolic antenna and diode
detector. This system produces a usable
Doppler signal for short ranges (50 ft or less)
with large caliber (57-280 mm) systems. For
longer ranges (165 to 320 ft) the M36
Chronograph has been used with filter
by-passed. To obtain results over ranges of up
to 10,000 ft, a modified HAWK CW
illuminator radar is applicable.
8-2.6 PHOTOGRAPHIC METHODS
Velocity also may be measured by means
of high speed (Fastex) motion cameras. A
typical setup is shown in Fig. 8-12. A board
several feet long with distance marks painted
on its side is placed parallel to the trajectory
as a distance reference. Parallax caused by the
board being behind the projectile axis will
cause an error in measurement. The correc¬
tion may be determined by knowledge of the
camera to object and camera to reference
distances, and simple trigonometry. Timing
marks normally are placed on the film for a
time reference. While less accurate and more
time consuming than previously described
measurement techniques, photography can
observe projectile integrity and launch charac¬
teristics as well as verifying velocity deter¬
mined by other systems.
8-13
Figure 8-9. Radar Velocity and Displacement Schematic
AMCP 700-230
Figure 8-IQ. Radar Velocity and Displacement
Schematic Using a Reflector
Figure 8-11. Simple X-band Interferometer
; *
' W
0 V,
fc* 1
-i.-v^W)3Hr 1PS*WW-
AMCP 706-238
t
i
s
i
i
Position Reference
AMCC 709-233
SECTION III
PRESSURE MEASUREMENTS
8-3 GENERAL
Next in importance to the measurement of
velocity is the measurement of internal
combustion pressures. Measurement of
approximate values of peak pressure may be
made with a copper crusher gage, however,
measurement of overall pressure-time infor¬
mation is really necessary for interior ballistic
and gun tube design. In addition to peak
pressure, the pressure-time curve will indicate
ignition delays, poor propellant burning
characteristics, excessively high muzzle exit
pressures, etc.
8-4 COPPER CRUSHER GAGE
The copper crusher gage as shown in Fig.
8-13 provides a measure of peak pressure
based on the measurement of compression of
a copper sphere or cylinder. It is used mainly
in proof firings, in development filings with
prototype weapons that do not _)erniit
modification for instiumencs, and as a simple
check on transient pressure measurements.
The pressures measured by these devices will
be from 5-25 percent below the actual
pressures read by the electronic gages when
the crusher cylinders are calibrated statically.
Obturating
Washer
Copper Crusher
Cylinder
Obturating Cup
Closing Cap
Rubber or
Neoprene Washer
Housing
'is ton
Pressure
Figure 8 - 13. Copper Crusher Gage
8-17
: ^wciw^-w A-nW*^-V-v gygnw ^ t * w ■ wa ^riwwf ^ IT . IJI 1
AM CP 706-238
Crushers ore available as internal gages for firings. The major disadvantages are the high
direct insertion into the propellant bed, which impedance output that requires a matching
are ejected from the gun on firing, and as amplifier, the transient distortion, and the
externally mounted gages on test guns. While poor low frequency response that may occur
their absolute accuracy is poor, it is possible if dampness or poor insulation effectively
to generate empirically a correction factor for lowers the output impedance. The lowered
a given ballistic system for use in routine output impedance effectively discharges the
firings and as a check and an indicator of gage circuit capacitance while the transient is
trouble in an electronic system. The correc- still being generated. A negative final (muzzle)
tion, however, may change from gun to gun pressure on the trace is indicative of
or ammunition lot to ammunition lot. excessively low impedance or leakage resis¬
tance in the gage circuit.
8-5 PIEZOELECTRIC GAGE The formula used for computing the
pressure constant P K is
A number of dynamic systems are in' use
for the measurement jf chamber pressures in p K - kC t /A { , psi-V* 1 (8-8)
recoilless systems. Transient pressures normal- *’
ly are measured by either a piezoelectric or where
strain gage type pressure transducer inserted
in the chamber of the weapon and recorded K = gage constant, lb-OttC)' 1
on an oscilloscope-camera or a magnetic tape
recording system. For normal recoilless C T - total input capacitance-including ca-
p systems having ballistic cycle times of pacitance of gace, iine, and ampli-
approximately 5 to 25 msec, the equipment fier-pF
should have flat frequency response over the
range of 0.5 to 10,000 Hz with response to A, = piston area, in?
DC (zero Hz) an advantage. In addition, phase
ihift must be low in this range to avoid where the input circuit of Fig. 8-14 is used,
distortion of the transient. The time constant RC T , where R is the
leakage resistance of the circuit including
The piezoelectric transducer offers certain amplifier input impedance, should be greater
advantages for measurement of pressure in than 100 times the ballistic cycle time to
recoilless systems. A piston of accurately assure good iow frequency response and low
known area converts the incident pressure to distortion. Values in the nature of 100
a compressive force on a piezoelectric crystal for R and 0.05 n? for C T are common for
or stack of crystals. Piston area of 1/6 in? is recoilless systems. Use of a capacitive
used for pressures of 3000 to 20,000 psi while feedback or charge amplifier with this type of
1/30 in? pistons cover the range of 12,000 to gage is also feasible where lines between the
90,000 psi. While barium titanate and gage and the amplifier are short (100 ft or
tourmaline crystals have been used, quartz less). This amplifier permits the calibration to
offers low temperature coefficient, high be independent of cable length,
stability, excellent high frequency response,
and reasonably high output. The gage is 8-6 STRAIN GAGES
self-generating, producing a charge propor¬
tional to incident pressure. The calibration Strain type pressure gages have the
changes little with aging or excessive overload- advantage of being capable of measuring static
ing, and the transducer readily can withstand pressure and, hence, may be calibrated by
the vibrations encountered in recoilless static hydraulic systems. A typical strain type
AM' ? 706-238
)
Following
Amplifier
pressure gage is shown in Fig. 8-15. It consists
of a ferrule or cylinder to which are bonded
foil or wire type strain gages. The cavity
normally is filled with silicone grease that
transmits the gun pressure to the ferrule,
expanding it and causing a change in gage
resistance. The strain gage makes up one or
more arms of a low impedance Wheatstone
bridge circuit that generates a voltage
proportional to its excitation voltage and the
pressure measured. This voltage is usually in
the millivolt range and considerable amplifica¬
tion is required to permit recording. This is a
disadvantage in field application where good
signal to noise ratios are sometimes difficult
to obtain. The strain type pressure gage also is
subject to changes in calibration factor when
overloaded by short duration transients. It is
advisable to verify its calibration frequently
or to replace the gage whenever excessively
high pressures or bridge imbalance is noticed.
All pressure gages have natural resonance
frequencies that can cause errors in measure¬
ment if the resonances are low in comparison
with frequency components in the phenom¬
ena under study. Step function response of a
gage should be investigated to ascertain that
ringing or overshoot does not occur in the
recorded output when subjected to a high rate
of change of pressure. A rapid pressure rise
for test of an individual gage may be
produced by a high pressure shock tube, by a
small closed bomb in which a primer or
detonator is fired, or by inserting pressure
gages in the barrel of a recoilless test gun
where they will be subjected to rapid pressure
rise as the projectile passes the transducer.
8-19
AMCP 706-238
SECTION IV
OTHER MEASUREMENT TECHNIQUES
8-7 STRAIN MEASUREMENTS
8-7.1 GENERAL
Recoilless rifle design normally stresses the
minimization o r weight with an associated
reduction in the thickness of chambers and
other components to that essential to safely
contain pressures anticipated. This requires
the accurate determination of dynamic
stresses on the system to assure that safety
margins are not surpassed. The use of strain
gages, brittle lacquer, “stresscoat” and similar
experimental techniques are therefore of
importance in the design of the prototype
system.
8-7.2 THE GAGE
The strain gage produces a change in
resistance proportional to the change in strain
in the surface to which it is applied. When a
known current is passed thiough the gage, the
output voltage change across the gage will be
proportional to strain occurring in the metal
as expressed by Eq. 8-9.
SJe = R g (GF)I , fi V per.u in.-(in.) _I strain
. ( 8 - 9 )
where
S v = output voltage change, as expressed
in juV
R g = gage resistance, £1
GF = gage factor, dimensionless
I - current passed through the gage, A
e = strain, pin.-On.J" 1
Since the strains encountered are dynamic,
it is not necessary to use a static bri* B c circuit
with the gage. A simple divider circuit such as
that shown in Fig. 8-16 may be used. The
capacitor C should be large enough to prevent
low frequency attenuation, considering th.
impedance of the amplifie: used. To produce
a constant current in the gage, it is necessary
that R be in the order of 20 times the gage
resistance R g . 120-fi or 350 -SI gages of the
“advance” wire (copper nickel) type are
usually used, having a foil resistance element
and epoxy backing. Since the gun may be
allowed to cool between rounds in a strain
test, it normally is not necessary to use high
temperature bonding techniques or tempera¬
ture compensation circuits. Outputs in the
order of millivolts are attainable for strains
encountered, using an excitation current in
the range of 10 mA. Strain gage application
techniques are covered in more detail in Refs.
5 and 6.
8-7.3 OTHER USES CF STRAIN GAGES
In addition to use in strain measurement,
strain gages also may be used on a chamber to
indicate pressure-time functions in a thin-wall
gun where attachment of pressure gages is
otherwise impossible. In some weapons, it is
possible theoretically to determine the rela¬
tionship between internal chamber pressure
and external strain at a point by knowing
chamber geometry. It is possible to determine
this experimentally^ by hydrostatic pressuriza¬
tion of the system or by the expedient of
firing a group of uniform charges First in
pressure instrumented test guns of the same
internal geometry. The same charge Fired in
the strain instrumented prototype should
produce a strain curve of the same shape
which can be related by an amplitude factor if
the average muzzle velocities coincide.
Strain gages also may be used to detect
passage of the engraving band on the
8-21
w:wms i
■<gw«S^BBBB8Wi Sw m mi
AMCP 706*238
+V
To Amplifier
Figure 8-16. Divider Circuit for Strain Gage
projectile at certain points on the barrel for
displacement-time on muzzle exit informa¬
tion.
8-8 ACCELERATION MEASUREMENT
8-8.1 GENERAL
The design of fuzing or sighting mech¬
anisms sometimes requires the measurement
of acceleration on the projectile or weapon
mounts. These measurements have been
accomplished using piezoelectric accelerom¬
eters designed specifically for shock applica¬
tions. Acceleration also may be determined
with reasonable accuracy by relating to
internal chamber pressure by Eq. 8-10.
PA = Wx (8-10)
where
P = internal chamber pressure, psi
A = bore area, in?
W = projectile weight, lb
x = acceleration, g’s
In most recoilless systems where bore friction
and pressure gradients are low, errors of less
than 5 percent are attainable by this method.
8-8.2 ACCELEROMETERS
It has been found possible to make
acceleration measurements by mounting pi¬
ezoelectric accelerometers in the nose of a
blunt test projectile. A usable signal is
transmitted out the bore over a low noise
coaxial cable and picked up by a cup on the
projectile nose during its travel down the
tube.
Also, accelerometers mounted in simulated
sights have been used to measure sight
AMCP 700-238
vibrations and, mounted to barrels, to
measure recoil forces as well.
weapon is mounted so that its center of
gravity is located as close as possible to the
center of gravity (which should also be the
geometrical center) of the pendulum frame.
8*9.2 MEASUREMENT OF RECOIL IM¬
PULSE
Displacement of the pendulum is shown in
Fig. 8-17, and direction of initial movement
may be determined visually using an indicator
mounted to the pendulum and a fixed scale,
or it may be photographed using a movie
camera. Recoil impulse I m may be deter¬
mined from Eq. 8-11.
4 = lb-sec (8-11)
where
W t • weight of pendulum with recoilless
rifle attached, lb
D - horizontal displacement, in.
Doppler radar (interferometer) also may be
used to produce projectile position-time
which may be double differentiated to obtain
acceleration. Some accuracy will be lost,
however, in the smoothing process.
8-9 RECOIL MEASUREMENTS
8-9.1 GENERAL
The measurement of recoil is of primary
importance in the evaluation of performance
of recoilless systems. The most used measure¬
ment is that of recoil impulse; however, it is
sometimes necessary to have information on
instantaneous recoil forces. Recoil impulse is
measured by recording the displacement of a
simple ballistic pendulum upon which the gun
is mounted. The triangular suspension system
of the ballistic pendulum reduces the effect of
torque on the pendulum displacement. The
g = acceleration due to gravity, ft-sec -2
r = period of pendulum, sec
Where visual methods are not suitable for
safety or other reasons, a displacement
transducer may be used to convert the
information to an electrical signal suitable for
recording. The transducer, however, must not
impede the movement of the pendulum.
The schematic diagram, Fig- 8-18, shows
another method used with good results for
measuring recoil displacement. A scale en¬
graved with reflecting marks spaced 0.1 to
0.2S in. apart reflects the light from a light
source to a photocell that produces a change
in current as each mark passes the cell.
Positive and negative directions of recoil ore
sensed by a difference in thickness of the
marks producing a different output signal.
8-23
—*V ^fl sw
AMCP 706-238
Photocell with Lens
Bmilimilllll
/£< Penlite
r~ y Bulb
Figure 8-18. Photoelectric Recoil Measuring Device
To Recorder
8-9.3 MEASUREMENT OF RECOIL 8-10.2 TECHNIQUES
FORCES
Recoil force may be measured through the
use of an accelerometer mounted along the
axis of the gun. Best results are obtained by
using strain gage type accelerometers having
low cross axis sensitivity, natural frequencies
in the order of 2000 Hz, and “g” ranges in the
order of several hundred “g’s”. The force at
any time may be calculated approximately
from Newton’s law, knowing the mass of the
pendulum and associated components. The
accelerometer output may be integrated
electrically by operational amplifiers to
obtain recoil velocity and position versus
time.
8-10 MEASUREMENT OF TEMPERATURE
8-10.1 GENERAL
In the design of recoilless rifles, several
aspects require the need for temperature
measurement at various points on the
weapon. Temperature rise measurements
during high rate of firing studies are used for
weight reduction studies and stress analyses
for barrel, chamber, and head sections of the
rifle. Interior ballistic studies require tempera¬
ture measurements on the interior of the
chamber wall and at the nozzle throat in
order to relate or determine quantities or
parameters such as mass rate of flow and heat
transfer.
As previously stated, temperature measure¬
ments are required at various points on the
weapon in several areas of design study. This
commonly is performed through the use of
thermocouples welded to the exterior of the
weapon. Differential (ungrounded) inputs are
required on the recording equipment to
prevent interaction between thermocouples
and noise pickup. Normally iron-constantan
junctions will suffice.
Measurement of temperature on the in¬
terior of the chamber wall and at the nozzle
throat is accomplished by the bore-surface
thermocouple shown in Fig. 8-19. The
thermocouple is made of an iron bolt through
which a nickel wire coated with nickel oxide
as an insulator is threaded. A layer of nickel
approximately 1 micron thick is plated across
the finely polished surface connecting the
bolt to the nickel wire and forming the
junction. It is necessary to replate the
thermocouple after each firing to insure
reliable performance.
8-11 PROJECTILE MOTION
8-11.1 YAW
Exterior ballistic studies require the mea¬
surement of projectile yaw. Precise measure-
8-24
AMCP 706*238
Figure 8-19. Bore-surface Thermocouple
ment of yaw requires the use of a series of
spark or motion picture cameras located in
two places at intervals down range. Reduction
of data will produce information on yaw
frequency and magnitude. Yaw may also be
measured with a lesser degree of accuracy by
the use of yaw cards, cardboard targets about
the thickness cf shirt cardboard placed at
intervals down range through which the
projectile is fired. Measurement of hole
elongation in the case of spin-stabilized
rounds and fin impression in the case of
fin-stabilized rounds along with measurements
of length of projectile (base to beginning of
taper or rotating band to fin) will give an
indication of magnitude of yaw while
frequency may be determined by observing
the distance between cards showing maximum
amplitudes and knowing the projectile velo¬
city at the cards.
3-11.2 SPIN
Spin may be measured in much the same
way as yaw. A spring loaded pin is inserted in
the side of the projectile which pops out
when the projectile leaves the barrel. The
angle of rotation occurring between two
successive targets is measured by observing
the impressions made by the pin in the
targets. It is necessary that two of the targets
be close enough together to assure that the
projectile has completed less than one
revolution between the targets. The spin rate
may be computed knowing the projectile
velocity at that position in the trajectory.
Increased accuracy may be obtained using
another set of targets spaced far enough apart
to permit several revolutions of the projectile
between targets.
Similar results have been obtained by
stretching thin wires (break circuits) ac.oss
the path of the projectile and noting marks on
a flat nosed projectile caused whan it struck
the wires.
A method of obtaining spin versus time
makes US'" of a spin sonde in which a
transistor oscillator with a loop antenna is
mounted in the projectile nose. The RF signal
AMcrmoa
is picked up by an antenna that parallels the
projectile flight. Signal strength is a function
of polarization of loop with respect to
antenna and produces a signal varying at two
cycles per revolution.
The current method for measuring spin is
by the induced emf technique. Projectiles are
magnetized and fired parallel to a passive coil.
This method has proved verj successful in
applications where the projectile is ferromag¬
netic. The method is very low in cost and
extremely accurate.
Spin also has been obtained on fin type
projectiles using the Doppler radar mentioned
before. In addition to the Doppler frequency
produced by velocity, a lower frequency in
the order of 200 Hz will be produced by a
slow spinning, finned projectile since the
Doppler return will be at a maximum when
any set of fins is parallel to the polarization of
the antenna. The Doppler frequency will
equal the spin rate times the number of fins
on the projectile
8-12 BLAST
8-12.1 GENERAL
The expansion of exhaust gases of a
recoilless rifle produces a pressure wave in the
surrounding air which is appreciably higher
than the pressure wave or blast from a closed
breech weapon of equivalent size. Pressure
wave measurement is necessary to determine
the boundaries of the zone from which
personnel must be excluded for safety.
The blast pressure curve ideally will be of
the shape shown in Fig. 8-20. The initial
compression wave of a few milliseconds in
duration will be followed by a negative
rarefaction wave of lower amplitude but
longer duration. Reflections from objects
surrounding the pressure measurement equip¬
ment may cause additional peaks; however,
the information of greatest importance is the
initial positive pressure peak.
8-12.2 BLAST GAGES
Pressure may be measured directly by the
use of blast pressure gages, which are basically
microphones of special design, or by deter¬
mination of shock velocity at a point and
computing the peak pressure. Gages are
located at a number of points around the
weapon under test at various heights, and a
plot is made of blast pressure versus position.
Blast pressure as low as 0.03 psi (140 dB) can
be detrimental to human hearing and, hence,
accurate measurements are of importance.
Pressures in the range of 30 psi (200 dB) may
be measured a few feet from the nozzle of a
recoilless weapon.
It is necessary that the gage used to
measure air blast pressures produce as little
restriction to the flow of the pressure wave as
possible while having a frequency response
high enough (in the order of 20 kHz) to
respond to the rapid rise of the blast
transient. Capacitor microphones having a
small diaphragm (0.25 in.) or pencil-shaped
piezoelectric blast gages have been used for
these mt^orements.
The pencil gags has produced the most
uniform results at blast pressures over 0.05
psi. It consists o' a ring shaped barium
titanate sensing element mounted flush with
the outer surface of the pencil shaped body to
provide a minimum effect in the air flow
across it. The gage is pointed (directly into the
origin of the wave. It must be mounted on a
fixture that does not resonate when disturbed
by the blast wave, and it should have a
streamlined appearance in the area of the gage
to prevent reflections from affecting the wave
front incident on the gage. In order to reduce
the effects of noise in the cable leading from
the gage to the recording equipment, the
cable should be buried, be of a nonmicro-
phonic type, and it may be necessary to
attach a low microphonic emitter follower
close to the gage to transform th? signal to a
low impedance for long transmission lines.
The output of the gage is a charge
8-26
' : }0J^^*^0km* •
v*
Figure 8-20. Typical Blast Waveform
AMCP 706-238
proportionaTto pressure in the order of S00
pC-(lb-in. 2 f 1 or about 0.25 V (psif 1 , if
connected directly to an amplifier by a short
cable. Parallel capacitors may be used, if
necessary', across the gage output to reduce
the signal level.
Blast pressure gages are calibrated in a
shock tube by comparison with a standard or
by measuring the velocity of the shock front
at the point of impingement on the gage and
computing pressure. A field check on
calibration is also possible by setting two
gages a known distance apart (in the order of
one to three feet) along the direction of
propagation of the wave and measuring the
time for the wave to travel between the gages.
The peak pressure P b of the wave may be
computed from Eq. 8*12 (see Ref. 7):
( 8 - 12 )
where
P 0 = atmospheric pressure, psi
V t = velocity of shock wave, fps
y =1.40 (specific heat ratio for air)
used may be oscilloscope, magnetic tape, or
equipment similar to that used for recording
of pressure and acceleration.
8-13 RECORDING EQUIPMENT
8-13.1 OSCILLOSCOPE
Most of the measurement devices previous¬
ly mentioned produce as an output either
time-varying analog voltages or pulses that
initiate or stop time interval counters. Where
only one or two channels of analog voltage
information are required, the osdlloscope-
polaroid camera method is most economical.
For calibration, a series of voltage steps are
applied to the vertical amplifier prior to firing
while the horizontal axis is modulated with a
time reference from a crystal marker oscilla¬
tor. Such a system is capable of providing
recording accuracies in the order of ±2
percent. The oscilloscope is triggered by an
electrical signal used to fire the gun or may be
triggered internally if the initial pressure rise
is not important for the test conducted. The
trace may be blanked at muzzle exit by a
pulse on the horizontal axis produced by a
muzzle break wire or a pressure gage located at
the. muzzle. A typical record obtained is
shown in Fig. 8-21.
Oo = speed of sound, fps
8-13.2 MAGNETIC TAPE
T = air temperature, *F
P w - partial pressure of water vapor at
temperature T, psi
P a = partial pressure of air at temperature T,
psi
Where a large number of channels of analog
information is to be recorded simultaneously,
the FM magnetic tape system is more
advantageous. ThL permits the reduction of
multichannel analog information on pressure,
strain, and other parameters at usable chart
speeds. Accuracies within i2 percent or better
are attainable if voltage steps and time
■narkers are recorded prior to firing and
carried through on playback.
The site chosen for the blast measurement
should be flat and free of obstacles that might
cause reflections. The recording equipment
Another item associated with the recording
equipment is a group of analog computer
operational amplifiers which may be pro-
8-28
Muzzle
Exit
t= 0
Figure 8-21. Typical Pressure-Time Curve
gnmmed to integrate pressure and accelerom¬
eter output to permit a real time recording of
velocity and displacement time information.
The blast effect of recoilless systems on
recording equipment as veil as safety
considerations normally require that the
recording equipment be placed s. me distance
away from the test site. This may require
cables of 500 ft or more which are sources of
noise and attenuation of the signal. High
impedance piezoelectric transducers will re¬
quire coaxial lines of very high leakage
resistance unless line driving amplifiers are
used at the test site. It has been found in
practice that mounting of outside termination
points in heated boxes will reduce leakage
resistance caused by moisture better than
attempting to seal such terminals from the
environment.
While strain gage lines are of low
impedance, care must still be taken to avoid
pickup from stray fields that will often
surpass the millivolt signal being transmitted.
Proper grounding of shields can best be
determined by experiment. Where calibration
resistors are used to calibrate strain gages,
additional leads usually are required to bring
the resistance effectively to ihe gage.
AMCP 706-236
SECTION V
GENERAL CONSIDERATIONS
In measurement of recoilless rifle param¬
eters as well as those of any other system, it is
essential to make use of redundant measure¬
ments whenever possible to indicate existence
of instrument malfunctions. The use of two
different methods of obtaining the same piece
of information is better than merely duplicat¬
ing equipment to obtain duplicate measure¬
ments. While velocity, for example, may be
measured by two successive velocity systems
(the one further down range should indicate a
lower velocity), an integration of a pressure
gage record will provide a cross-check as well.
By use of formulas and empirical relation¬
ships, pressure may be checked against strain,
terminal velocity against muzzle velocity,
acceleration against pressure, etc.
Equipment should be calibrated periodical¬
ly and, in addition, a standard round of
ammunition should be fired immediately
prior to a test to check out equipment under
dynamic conditions to avoid loss oi data in an
important test scries.
Records should be kept during each firing
program of instruments and transducers used,
calibration factors, setup methods, and
unusual results noticed in addition to actual
test data. When at all possible, photography
should be used to show test setups as well as
being applied as a measuring technique. While
quantitative data are sometimes difficult to
obtain by photographic means, high speed
photos of the weapon and projectile during
firing are an invaluable tool in determining
causes of malfunctions.
Certain aspects of measurements, safety
considerations, test procedures, and the like
are covered in more detail in Refs. 1 and 6,
and in texts of test equipment manufacturers’
brochures on measurement equipment. The
reader is encouraged to refer to these for
further information.
REFERENCES
1. AMCP 706-181, Engineering Design Hand¬
book, Explosions in Air, Part One.
2. AMCP 706-150, Engineering Design Hand¬
book, 3allistics Series. Interior Ballistics of
Guns.
3. BRL Report 968, Ballistic Studies with a
Microwave Interferometer, Part 1, 1955.
4. BRL Report 1006, Ballistic Studies with a
Microwave Interferometer, Part II, 1957.
5. Measurement Engineering, Stein Engineer¬
ing Services, Inc., Phoenix, Arizona, 1964.
6. NPG Report 1241, Development of a
Pressure Time Recording System for the
20 mm Anti Aircraft Gun, U S Naval
Proving Ground, Dahlgren, Virginia, 1954.
7. H. W. Liepmann and A. Roshko, Elements
of Gasdynamics, John Wiley and Sons, N.
Y., 1957, p. 64.
8-31
mMCP 706*238
PART THREE DESIGN
CHAPTER 9
BASIC DESIGN CONSIDERATIONS
SECTION I
INTRODUCTION TO DESIGN CONSIDERATIONS
9-1 ADVANTAGES OF RECOILLESS
RIFLES
The major advantage of a recoilless rifle
over closed breech weapons and the reason
for its development are found in the light
weight of this weapon system; the ratio of
projectile kinetic energy to system weight is
very favorable for a recoilless rifle compared
to a dosed breech weapon. Jn small caliber
recoilless rifles, less than 100 mm, this
advantage is more pronounced because the
recoilless principle produces a portable gun,
capable of being shoulder fired with a
probability of hit at battle ranges quite
comnr able to that obtained by the conven¬
tion '*osed breech gun. The light weight of
the rtcoilless rifle is ■> iade possible mainly by
the cancellation of recoil, and, to a certain
extent, by the high piezometric efficiency
inherent to recoilless rifles.
Recoil is cancelled in the recoilless rifle by
providing sufficient propellant to produce
gases to accelerate the projectile forward and
to discharge rearward through a nozzle such
that the impulse of gases discharged equals
the projectile momentum. The result is that
substantially no recoil impulse h imparted to
the rifle, mid the weapon does not require any
of the heavy recoil mechanisms that a
conventional gun would require, i.e., no heavy
carriages and no pneumatic or spring recoil
systems ordinarily present in the weapon
system are needed. Jkcwise, the mount for
the rifle is made relatively light in comparison
with the mounts required to support the
heavier conventional guns.
The weight saving realized by use of a
recoillcss rifle is shown in Table 9-1.
The data in this table clearly show the weight
advantage of a recoilless rifle over a
comparable closed breech gun. Comparison of
the 75 mm system shows the closed breech to
be 8.6 times as heavy < s the comparable
recoilless rifle.
9-2 IMPORTANCE OF SYSTEM DESIGN
APPROACH
The usefulness of the recoilless rifles,
because of their light-weight, lies in the ability
to give infantry the capability of defeating
small fortifications, armored targets, and area
targets. V/hen necessary, it is possible to
hand-carry the 57 mm, 75 mm, and 106 mm
and even the 120 mm and 155 mm, recoilless
rifles for short distances, over average terrain,
in order to support combat rifle units in any
situation. Accordingly, it is necessary that
human engineering, maintainability, and reli¬
ability be considered in the design approach
i.e., these factors together with the goal of
providing a specific terminal effect form the
criteria which are studied and weigned in
designing the recoilless rifle as a totally
integrated system. The roles tiiat human
engineering, reliability, and maintenance fac¬
tors play in the design of recoilless rifles aie
AMCP 706-238
TABLE 0*1
COMPARISON OF 75 snm RECOILLESS AND CLOSED BREECH WEAPON SYSTEMS
Projectile
Muzzle
Weapon
System
Ratio Projectile
Kinetic Energy to
Weight,
Velocity,
Weight,
Weapon Weight,
Weapon
to
ft-tec 1
to
ft
75 nrtrti, M20 Recoilless
13.1
1000
168
1210
75 mm, M1A1 Howitzer
13.4
1000
1440
144
discussed in the last three sections of this
chapter. The remaining part of this paragraph
will describe briefly the various components
of the recrilless rifle and their interrelation*
ship with other components of the system.
The two largest components of the
recoitless rifle, which together account for
approximately two-thirds of the rifle weight,
are the gun tube (or barrel) and chamber. The
gun tube for the recoilless rifle is similar to
the barrel of a standard artillery gun. The gun
tube h?s a rifled bore and fires a conventional
projectile that incorporates a standard fuze.
The ammunition used in the recoilless rifle is
of a special type in which the cartridge case is
perforated to permit the easy escape of the
propellant gases (see Fig. 9-6 and 11-9). As in
the case of the standard artillery projectile,
the perfoiated cartridge case remains in the
chamber during firing, the projectile being the
only component that is ejected from the
weapon. The chamber of the recoilless rifle is
considerably larger than the chamber of the
similar caliber conventional gun and is closed
only partially at the rear by the breechblock.
The breechblock often contains the nozzle
which allows for the rearward release of the
propellant gas, and contains part of the firing
mechanism for detonating the cartridge
primer. The remaining components of the
recoiliess rifle are the triggering mechanism,
the mount, and the sighting mechanism. The
design and types of recoilless rifles are
described in more detail in Chapters 10
through 13 of this handbook.
9-3 DESCRIPTION OF VARIOUS WEAPON
CONFIGURATIONS
3-3.1 BASIC PRINCIPLE
The basic principle underlying the opera¬
tion of the recoilless weapon is the principle
of “Conservation of Momentum”. Chapter 6
of this handbook contains a detailed presenta¬
tion relating to cancellation of recoil includ¬
ing the basic principle on conservation of
momentum. Ih application to recoilless rifles
balance is achieved by producing an impulse
of the gases discharged through the nozzle
equal to the effective momentum of the
orojectile and the gases accelerating the
projectile. After shot ejection, the impulse of
the gases discharging through the nozzle may
exceed that of the gases discharging titrough
the muzzle and a slight forward balance may
result. However, it is the practice to select the
nozzle throat area such that the weapon
recoils slightly rearward. Thus, as the nozzle
erodes as a result of successive firing, the
rearward unbalance becomes less and with
sufficient erosion will become forward. The
contribution of the recoil unbalance after
shot ejection is quite small and is compen¬
sated in the initial selection of throat area.
9-3.2 THfc DAVIS GUN
The application of the principle of the
recoilless gun dates back to World War I,
when it was incorporated in the basic patent
of the Davis gun. As shown in Fig. 9-1, the
9-2
AMCP 708-238
Figure 9-1. Davis Gun Mounted on WW I Martin Bomber
Davis gun consisted mainly of a straight tube plastic burst or blcw-out disc in its base. As
that simultaneously ejected a projectile from shown in Fig. 9-2, the chamber is much
the muzzle and a heavy lead shot with low smaller than that of a comparable US
velocity from the breech of the weapon (Ref. recoilless rifle (Ref. 1), see Fig. 9-6. The
1). Considered for use as a possible weapon resulting difference in operation was that the
for airplanes, the Davis gun had the early German and Russian rifles operated at
disadvantages of being both awkward and higher chamber pressures with short barrels,
shooting the lead shot rearward. As a result, whereas US and British guns employed lower
the gun was abandoned by the Army and the chamber pressures with longer barrels.
Navy.
An obvious disadvantage of the German
3-3.3 RUSSIAN AND GERMAN DESIGNS and Russian guns was the discharge of solid
particles through the nozzle. Another, more
The early Russian and German recoiliess important, disadvantage of these guns was the
guns were different from the US and British reduced efficiency of their projectile. Since
designs in that they did not use perforated these guns operated at higher chamber
cartridge cases. Instead, they preferred using a pressures, the projectiles had to be designed
conventional type of cartridge case with a to withstand the higher acceleration forces.
AMCP 706-23*
When the projectile wails and base are made
thicker to meet the higher strength re¬
quirements, the amount of high explosive that
will fit in the projectile cavity is reduced. If
the projectile base thickness is increased to
withstand the higher acceleration stresses, it is
also necessary to reduce the amount of metal
at the nose of the projectile, so that the
acceleration stresses due to the weight of the
nose are not being transmitted to the base.
The resulting projectile design, with its
uneven distribution of metal and reduced
explosive content, results in a projectile that
is less efficient with regard to both explosive
charge to projectile weight ratio and fragmen¬
tation.
9-3.4 THE BURNEY GUN
During World War II, British designer Sir
Dennis Burney applied the recoilless principle
to a series of weapons which were de\ Moped
for the British Government. The first of the
Bumey guns was the 95 mm R.C.L. Twin Jet
Gun which is shown in Fig. 9-3 (Ref. i, Vol.
VI). Sir Bumey incorporated the use of
multiple nozzles with large expansion ratios.
The divergent nozzles were mounted on the
sides of the breech ring. This allowed the use
of a standard sliding block type breech
mechanism, similar to that used on a 95 mm
howitzer, which would accommodate a
conventional cartridge case flange. In this
manner, loading, firing, breech opening, and
cartridge extraction functions remained the
same as for the standard 95 mm howitzer.
Recoil adjustments were made through the
use of interchangeable throat rings of varying
diameters. Although the Bumey guns were
rccoiliess, the total weight saving that could
have been made was not achieved. For
example, the 95 mm Twin Jet Gun shown in
Fig. 9-3 weighs 700 lb without any carriage as
compared with the US 106 mm, M40
Recoilless Rifle which weighs only 274 lb.
In 1944, a series of recoilless mortars 60
mm, 81 mm, and 4.2 in. applied the current
technology to these high angle fire weapon
systems. Many experimental models were
designed and built. Figs. 9-4 through 9-6
illustrate a representative technology in these
three tube calibers.
9-3.5 THE HYBRID WEAPON
It was not until the late 1940’s that the
principles of the front orifice recoilless rifles
were studied in detail. The front orifice rifle,
which is shown in Fig. 9-7 (Ref. 2), has the
distinct feature that the projectile covers the
nozzle ports at the beginning of the ballistic
cycle. The propellant charge is confined to
the closed chamber which provides for good
ignition. The advantages of this design as
compared with rear orifice weapons are:
1. Unperforated cartridge cases can be
used.
2. Good ignition characteristics are at¬
tained.
3. Propellant loss through nozzle is negligi¬
ble.
4. Nozzles of large expansion ratio can be
used without increasing weapon length.
The disadvantages of the front orifice rifle
are:
1. An initial rearward momentum is
generated until nozzle ports are uncovered.
2. Added weight due to heavier chamber
design, longer nozzles, and added gas ducting
system.
9-3.6 SIDE-LOADING CONFIGURATION
Fig. 9-8 (Ref. 3), comprises sectional views
of a proposed side-loading, magazine-fed,
blow-back operated, repeating, recoilless
system. The basic components of the system
are the guide tube, the barrel and the
box-type, and spring-loaded magazine
mounted below the weapon. A magazine latch
AMCP 706-238
BURNEY 9SMM R.C.L. TWIN JET GUN AND CARRIAGE
(REAR VIEW)
BURNEY 9SIM R.C.L. TWIN JET GUN AND CARRIAGE
(REAR VIEW)
Figure 9-3. Burney 95 mm R. C. L. Twin Jet Gun and Carriage
Figure 9-4. 60 mm Recoilfess Mortar
AMC97C3-238
Figure 9-5. 81 mm Recoilless Mortar
is provided for opening and locking the been loaded, closing the magazine, releasing
magazine in the closed position. A detent the magazine detent, and allowing the barrel
handle actuates the mechanism for holding to return. This action places the first round in
the round in place during loading. A cocking the chamber, and the weapon is ready to fire,
handle is provided for initial cocking of the As each round is fired, the barrel automatical-
weapon. ly cocks, the weapon is loaded, and the barrel
returns to the “ready-for-firing” position.
After the last round is fired, the barrel is
Operation of the -system includes initial locked forward and the magazine may be
cocking of the weapon, which involves pulling opened for reloading. Closing the magazine
the gun barrel forward after the magazine has automatically releases the barrel.
9-8
Figure 9-64.2 in. ftecoit/ess Chemical Mortar
. III-JIIIIQ .1 . II.-I
AMOP 706-238
Ifcag
''vWvr
: ^-V. %:;,= ■
. »•'
=
**:$&
•SS^'v^'
■' x -h*.
. .
:<&«,; ■ >r.
Figure 9-7. T135 Front Nozzle Rifle
The important characteristics of this type
of a repeating recoilless system are:
1. Relatively simple and compact mech¬
anism
2. Unusually light weight (total unloaded
weight approximately 300 lb)
3. Variety of mounting possibilities
4. Ease of loading for ar automatic use.
The most important characteristic of the
side-loading design is the feature for which it
is named: Side-loading. This feature is
convenient for vehicle-mounted applications.
Because of its compactness, it can be enclosed
completely to prevent possible jamming due
to environmental and combat exposure.
The 105 mm, T237 Rifle, Figs. 9-9 through
9-11, was developed to provide a vehicle for
investigation of weapon mechanism problems,
sealing problems, ballistic problems, and other
concomitant problems associated with high
rates of fire. It is a semiautomatic, fire
chamber revolver type recoilless rifle capable
of firing S rounds of fin-stabilized 10S nun
HEAT ammunition within 12 sec. Total
weight of the system is 710 lb exclusive of
ammunition. Original models of the system
were electrically operated so that power
sources th.it are normally available on board
full-tracked, self-propelled vehicles could be
used. A gas operated option was developed
along with a mechanical firing system that
could be easily substituted for the electrical
system.
Unique assembly and disassembly features
were designed into the system so that
assembly and disassembly could be accom¬
plished in minimum time.
9-3.7 CONFIGURATION WITH PERFO¬
RATED CARTRIDGE CASE
One of the most important features of the
recoilless rifle system is the use of the
9-10
Figure &8. Side-loading Configuration
NOTCHES WHEEL
TU6E
IMtftL SMKft
AMCO 706-238
perforated cartridge case. As shown ir. Fig.
9-12 (Ref. 4), there are hundreds of small
openings, with a thin cover that disintegrates
to provide for the flow of the generated
propellant gas. As the combined area of these
perforations greatly exceeds the bore or
throat area, there is little resistance to the
flow of gas from the cartridge case. Thus, the
case is made considerably smaller than the
chamber. This, in turn, minimizes fluctuations
in burning by insuring ignition at high loading
density and combustion at correspondingly
low loading density.
In order to be able to use several types of
ammunition in a particular recoilless rifle or
to be able to compensate for the effect of
nozzle erosion and extreme ambient condi¬
tions on rifle performance, it is necessary to
incorporate a recoil compensating device into
the weapon system. Fig. 9-13 (Ref. 5), shows
a recoil compensating ring installed in place in
an M40 Rifle. The purpose of the ring is to
enable the nozzle area to be adjusted until the
recoil for a particular rifle-round combination
is within the allowable tolerance. It is thus
possible to adjust for any changes in the
physical characteristics of the nozzle or type
of ammunition.
Fig. 9-14 (Ref. 6) shows the use of a
perforate^ cartridge case with a blow-out disc.
The blow-out disc is one of the most
nnportant items ir Lhe central nozzle type
recoilless rifle. The blow-out disc controls the
amount of combustion which takes place
prior to gas discharge by confining the
propellant gas until the disc ruptures, and,
hence, exerts considerable influence on the
amount of unburned propellant loss through
the nozzle during the initial stages of burning.
The propellant gas generated passes through
the cartridge case perforations into the
annular chamber. After rupture of the
blow-out disc, the gases pass from the
chamber, back across the cartridge case wall,
and out through the central nozzle. Blow-out
discs having a thickness of 1/4 to 5/16 in.
have been made from paper-base phenolic
materials for efficient use of propellant.
9-3.8 SPECIAL CONFIGURATIONS
Fig. 9-15 (Ref. 7) shows a typical
configuration of a fin-stabilized projectile
with the propellant attached to the boom.
The propellant in this figure is pimple
embossed sheet propellant in the form of
circular discs with holes in the center, for the
purpose of stacking on the projectile boom.
The tapered stack of discs provides for proper
ignition of propeliant charge. The technique
of attaching the propellant io the projectile
also has the advantage that the metal or
frangible cartridge case necessary to contain a
granular form of propellant may be elim¬
inated. This caseless charge then results in
redu< round weight, greater economy in
manufacturing, and elimination of case
ejection after firing.
In many weapon systems it is often
desirable to exund the maximum range. Fig.
9-16 (Ref. 8) shows a method of increasing
the range of recoiliess rifles through the use of
a rocket assist. A solid propeliant rocket
motor is attached to the rear of the warhead
compartment to provide the added energy for
attaining the extra range.
One of the last series of recoilless weapon
systems to be developed was the DAVY
CROCKETT. As shown in Fig. 9-17, the
DAVY CROCKETr weapon fired a large
caliber projectile from a smaller caliber
recoilless rifle. The main characteristics of tills
type of weapon arc that the propellant gases
act upon a ‘‘pusher'* tube (spigot) which in
turn pushes the oversize warhead that is
accelerated completely outside oi tnc gun
tube. Tire use of a large, low pressure gun is
then eliminated along with its associated
problems of ballistic reproducibility.
9-4 DISADVANTAGES
The main disadvantages of a recoilless rifb
9-15
Projectile
\
Blow-out Disc
Perforated Cartridge Case
Figure 9-14. Sketch of Perforated Cartridge Case With Blo.v-out Disc
AMCP 706-230
9-21
: * v; >. -.v# ~
AMO* 706-238
are the backhlast, its associated flash, and
round-to-round dispersion. Backblast is an
inherent characteristic of the recoilless rifle
desdgn-the result of the escape of propellant
gas to the rear of the gun. When using a
recoilless rifle, it is necessary to protect both
personnel and materiel for many yards to the
rear of the gun. For example, the danger zone
of the 57 mm Ml 8 Rifle is a cone extending
50 ft to the rear of the weapon with a base 40
ft wide. In addition, personnel within 100 ft
to the rear of the breech should not face the
weapon because of the danger of flying
particles thrown up by the blast action. While,
in some cases, backblast may prevent the use
of a recoilless rifle, altogether, many of the
requirements can be met without much
difficulty.
Associated with backblast are the undesir¬
able effects of flash and smoke which serve to
reveal the weapon position to the enemy.
Whereas, the magnitude of backblast is
determined by the projectile energy and,
therefore, is incapable of considerable reduc¬
tion, reductions in smoke and flash intensity
have been made by the use of “smokeless”
and “flashless” propellants.
Another disadvantage of a recoilless rifle is
the inefficiency because of the additional
propellant charge required to make the
weapon recoilless. The ammunition weight
and size per round ore, therefore, greater for
the recoilless rifle than for a closed-breech
weapon of similar caliber. For example, in the
case of 75 mm guns, the cartridge for the
recoilless rifle would require almost 3 lb of
propellant as compared to 1 lb for a howitzer
cartridge (Ref. 13). A 105 mm recoilless
cartridge requires 8 lb of propellant as
compared to 3 lb for the same caliber
howitzer. Because of the extra propellant
charge, the recoilless rifle cartridge case is
longer and heavier than the dosed-breech
type cartridge case. However, the extra weight
is offset somewhat by the fact that recoilless
rifle projectiles do not have to be designed to
withstand the higher acceleration forces io
which a comparable howitzer projectile would
be subjected during firing.
Low thermal capacity of the weapon
system due to the light weight of the
recoilless rifle, as compared with the heavier
conventional guns of the same caliber, limits
the rate of fire. Consequently, under high
sustained rates of fire, the temperature of the
recoilless rifle rises fairly rapidly to levels at
which the yield strength of the gun tube
material may fall below the safe design value
and where cook-off of the cartridge may
become a problem (Ref. 3).
Although initial development of recoilless
rifles had as their objective in the United
States giving the infantry antiarmor fire¬
power, later objectives included adaptation to
lightly armored combat vehicles for airborne
forces. These objectives also lead to considera¬
tion of automatic and remote feeding, nozzle
gases ducting, and salvo systems. Figs. 9-18
through 9-31 illustrate a variety of fielded and
proposed configurations.
9-23
’.■Wl t23MiV
vy* ;
■»
■'IWWPWfl
iiiiimfiiTrriuTanjvnipnn bbbb
AMCP 7D6-23S
SECTION II
HUMAN ENGINEERING
9-5 INTRODUCTION
It is imperative that each new weapon
design be studied and tested extensively in
order to insure its safety and efficient
functioning under the control of human
operators.
From the safety standpoint, rccoilless rifles
have the unique requirement in that consider¬
able attention must be devoted to the effects
of baclcblast, noise, smoke, and flash. In
addition, operational effectiveness is depen¬
dent upon other features of the weapon -such
as the lack of recoil, light weight, and the
psychological effects of blast, smoke, and
flash. Also, the tactical and psychological
advantages of placing artillery-type weapons
in the hands of front-line infantry must be
considered.
In order to make a concerted effort to take
the capabilities of the human operator into
account so as to increase the effectiveness of
recoilless weapons, the Engineering Psycholo¬
gy Division of Frankford Arsenal (Ref. 9)
divided the scope of the problem into four
areas. These channels of study were
1. Analyses of the human factor aspects of
previously developed systems, in order to
determine whether any important steps had
been omitted or whether shifts in emphasis
might have produced a better weapon system.
2. Studies of the physical features of
present experimental and standard weapons,
bringing to bear upon them human factor
data from physiology, anthropometry, and
psychology.
3. Analyses of recoilless rifles and person¬
nel functioning as a weapon system, to
improve safety and efficiency.
4. Studies of the human factors evident
from the first three investigations which
affect the probability of a first round hit on a
selected target and reduce recognition-to-hit
time.
9-6 PRIMARY FACTORS
SMS.1 THE MAN USING THE WEAPON
The human engineering design considera¬
tions for the man using the weapon can be
grouped into the following areas:
1. Weapon and ammunition carrying
2. Breaking-out ammunition
3. Emplacing, moving, and re-emplacing
rifle
4. Loading and unloading rifle
5. Detecting and choosing targets
0. Ranging, laying of rifle, and firing.
Since recoikless rifles are infantry weapons,
the problem of portability is very important.
The weight of the rifle will determine whether
it is capable of being carried by one or two
men and how it is to be carried. A rifle
weighing less than 3S lb (Ref. 14) can be
efficiently carried high on the back wltile
heavier rifles are more efficiently carried low
on the back. Rifles weighing up to 70 lb are
capable of being carried by one man for a
short distance but will have to be carried by
two men for longer distances. The type of
carry employed for a specific rifle must be
taken into account in the design of the
weapon. For example, a two-man breech-
muzzle carry, which is less fatiguing than a
side-by-side carry, necessitates either per-
AMCf> 706-238
manent rigid handles attached to the rifle or
the use of two web straps encircling the barrel
and chamber of the gun.
A potentially critical situation arises with
the covering and uncovering of r iflc, and
breaking out the ammunition. Ip ,.i case of a
surprise attack, it is necess? j to rapidly
uncover the rifle and break out the
ammunition even under conditions of incle¬
ment weather. An important consideration in
this area is the ability to perform these tasks
while wearing arctic clothing and mittens.
Because of the versatility of the smaller
caliber recoilless rifles-capable of being fired
either from the shoulder, from an integral
mount or from a number of auxiliary
mounts-the gun emplacement task is another
important factor which must be included in
human engineering design considerations.
Depending upon the type of mount-integral
or separate-different unfolding, locking,
unscrewing, and adjusti% operations may
have to be performed during the emplacement
of the gun. As such, all locking or adjustment
knobs should be easily accessible and large
enough to permit rotation, and all extension
operations of bipod arms easily made under
all conditions.
Of all the operations of the recoilless rifle,
the loading and unloading of the rifle aie the
most critical of the human engineering aspects
that must be considered. The loading and
unloading of a rifle basically consists of
unlocking and opening of the breech,
inserting the round, closing of the breech, and
extraction of the round. One of the essential
requirements in this operation is that the
handles on the breech be designed so that the
assistant gunner (loader) is not required 10
have any part of his body pass behind the rifle
when opening and closing the breech. This
will depend, of course, on the proper
placement and orientation of both breech and
chamber handles, and the manner in which
the breech opens. In order to prevent fatigue
of the loader and the resulting slowdown of
round loading, it is also desirable to have the
breech operations performed by horizontal
pushing or pulling movements relative to the
assistant gunner's chest since a man can exert
more force by a push or pull than by a
sidewise motion.
A specific human engineering design
consideration relative to the round insertion
operation is to insure that the rotating bands
on the ammunition automatically engage the
lands and grooves of the barrel to eliminate
the delays that may result from indexing. The
one aspect of the extraction process which
must be considered is to allow sufficient
clearance, so that the loader can make
complete extraction when wearing arctic
nuttens, after the positive extraction of the
round by the extractor device in the
breechblock.
Important safety requirements necessitate
that the gunner’s safety indicator move
automatically to the "safe” position when the
breech is opened and that it be manually
returned to the “fire" position after the
breech is closed before the weapon can be
fired. Hie indents of the safe and fire
indicators, stamped “S" and “P*. respective¬
ly. should be visible under all light conditions.
The human engineering design considera¬
tions involved during the design of a recoilless
weapon may be subdivided into four areas: (1)
pistol grip and trigger, (2) shoulder rests and
recoil, (3) monopod handle, and (4) sight.
The important design factors of the pistol grip
are the trigger pull force be not more than
about 7 lb per finger and tliat the trigger
travel be not more than 0.5 in. Any travel
longer than 0.S in. causes uncertainty on the
part of the gunner or may require the gunner
to change his grip to exert more pressure on
the trigger.
Should : r rests should be adequately de¬
signed dimensionally for usage with both light
and arctic clothing. Consideration also should
be given to the possible use of canvas-covered
9-38
AMCr7»2N
pads attached to areas which contact the
shoulder.
For shoulder-fired recoilless rifles, a desir¬
able design consideration in the position of
the monopod handle is that the monopod and
pistol grip be in the same vertical plane. This
allows the gunner to brace the upper part of
his left arm against his body while grasping
the monopod when the weapon is fired. The
result is increased steadiness of holding the
rife, with consequent better aiming.
It is important that the sighting be
positioned sufficently forward, so that when
the rifle is fired from the prone position, the
gunner is clear of the blast area without being
extremely cramped. A second consideration is
the use of a sL.idaidized sight, since
variations in sights may lead to a disruption of
performance when a gunner switches weap¬
ons. Sufficient head clearance also should be
allowed between the sight and face shield of
the rifle so that the gunner can fit his eye to
the eyepiece of the sight while wearing a
standard combat helmet. For further informa¬
tion on some of the human engineering
considerations given to the first recoilless
rifles, the reader is referred to Ref. 10.
9-6.2 FIELD SERVICING
Field servicing, or maintenance, is the
service performed at the organizational level
to keep the weapon in proper firing
condition. This servicing consists of periodic
inspections, cleaning and lubrication, minor
adjustments, and replacement of parts that
are worn, broken, or otherwise unserviceable.
Because of the mobility requirements of the
recoilless weapon, the amount of tools, test
equipment, and supplies is limited to the
organizational level, dince the maintenance
skills at the organizational level arc of a
limited nature, the design engineer must plan
appropriate preventive maintenance and ser¬
vicing procedures.
All parts of the rifle should be easily
inspected for damage or traces of corrosion
and then allow for ease of cleaning and
lubricating. All damaged parts should be
capable of replacement with a minimum
amount of hand tools, i.e., removed from the
weapon simply by loosening screws and nuts
or by rotation of a locking knob, it should be
emphasized that even the simplest of these
types of operations should be capable of
being performed while wearing arctic cloth¬
ing.
&4L3 MANUFACTURING PERSONNEL
The basic human engineering design con¬
siderations to be observed during the course
of manufacturing of the recoilless rifle are
similar but not limited to those associated
with the operation ard servicing of the
weapon. Additional consideration must be
given to the safety of the personnel loading
and asset: Ming the round of ammunition, and
to the complexity of the individual assembly
operations. Because of the nature of the
round of ammunition, it is necessary that
each assembly operation be as safe as possible,
and performed under controlled conditions of
environment.
In addition, each assembly and loading
operation should be as simple as possible so as
to avoid any possible assembly error while
keeping the cor* of the assembly to a
minimum. The order of assembly operations
also must be given special consideration, since
the effect that one operation may have on
another can result in errors uncontrollable by
the assembly personnel. The early difficulties
encountered in the original design of the
fin-stabilized HEAT ammunition for the I OS
mm BAT weapon are examples of some of the
uncontrollable problems that can arise be¬
cause the pertinent aspects and order of
assembly were not considered when me
design was established. For example, the
necessity of positioning and retaining the
propellant to the rear of the cartridge case
complicated the loading process so that the
loader inadvertently would introduce propel-
9-39
, v i — .acgEgggarawc sg a
AMCPXW-238
lant grains in the primer cavity unless he was
extremely careful. This placed unnecessary
responsibility on the loader for what is a
normally simple operation. The most impor¬
tant consideration to be given here is that
there should be dose coordination between
the design organization and the manufactur¬
ing and loading facilities during the develop¬
ment stage of the weapon in order to insure
that the proper input of the human
engineering factors is taking place.
9-7 HUMAN FACTORS ENGINEERING
EVALUATION
Since the function of human engineering is
to increase the efficiency of man-machine
system in order to evaluate the superiority of
one design over another, it is often necessary
to test the design concept with the use of a
mock-up and test personnel. One of the first
examples of a human engineering test on a
recoilless rifle was a time study of the
registering of pre-engraved projectiles in the
rifling (Ref. I, Vol. IV). A time study was
made of the act of chambering complete
rounds, since there might be difficulty in
lining up the pre-engraved rotating band with
the barrel rifling Twenty rounds of ammuni¬
tion were divided into two equal lots of 10. In
one of the lots, the pre-cngraved rotating
band was cut away completely so as to offer
no obstruction to chambering. Three opera¬
tors were chosen to perform the test, only
one having had any previous experience with
a recoilless rifle. A time study was then
performed of chambering ten pre-engraved
rounds and then ten plain rounds. Each
chambering operation consisted of picking up
the round, chambering it, closing rnd locking
the breech, opening the breech, and extract¬
ing the round. Timing was done with stop
watch by an independent observer. This
pattern was repeated until fifty rounds of
each lot had been chambered. It wi found
that the use of pre-engraved project ; took
approximately 25 percent longer to jgister
than the plain rounds.
This result led to the eventual incorpora¬
tion of automatic indexing devices in the
projectiles, such as indexing buttons and
springs, which reduced the time required to
line up the pre-engraved rotating band with
the lands rad grooves of the rifling. For a
round with indexing buttons, it is merely
necessary to place the round in the chamber
so that the projectile is started down the tube
and then drive the round home with the heel
of the hand. The indexing button operates by
compressing into the projectile when it strikes
a land, and then expanding when it hits the
rifling groove. After engaging the groove,
these buttons force the round to rotate so
that the pre-engraved band lines up with the
barrel rifling as the round is chambered and
completely inserted. Thus, as a result of these
tests, a slight redesign of the pre-engraved
projectile enabled registering time to be the
same as that of a round without the
pre-engraved rotating b\nd.
94 AREAS OF APPLICATION
Some areas of specific application of
human engineering have been described in
par. 9-6. However, at any instance of
man-weapon interface-i.e., during weapon
carrying, emplacing, loading, firing, main¬
tenance, testing, manufacturing, assembly,
etc.-it is necessary to insure personnel tue
safety while trying to attain the highest
efficiency of the recoilless weapon system.
For further information, the reader is referred
to Ref. 10 and Ref. 14.
94 SPECIFIC RESPONSIBILITIES
The responsibility of the human engineer¬
ing group is to insure that the design engineer
has considered the human factor of the
weapon system, both from the standpoint of
safety and from the standpoint of operational
effectiveness of the weapon. This responsi¬
bility begins with the analysis of the initial
design and continues with a detailed opera¬
tional and performance study of the proto-
AMC? 70*23®
type weapon. It coordinates the initial
manufacturing and assembly operations with
the design organization, and complies the
feedback of user comments on the weapon
during actual field experience.
In the collection of data from these various
analyses, the human engineering group is
responsible for identifying specific problem
areas and identifying the specific factors that
should receive highest priority. By contin¬
uously monitoring all phases of the develop¬
ment program, this group assures that the
human engineering function is incorporated
into the design of the weapon system and will
not have to be compromised, “added on**, or
“built into** the weapon at a later date with
the resulting high cost.
9-41
AMCP 706-238
SECTION III
RELIABILITY
9-10 BASIC PRINCIPLES
Reliability is expressed simply as the
probability of a successful operation in the
mode for which it was intended* In the case
of a recoilless rifle or other weapon system,
this successful operation is the scoring of hits.
While the design engineer keeps in mind th»s
main purpose, he must consider such various
constraints of secondary nature as cost, light
weight, low silhouette, portability, and ease
of maintenance and operation -all of which
might affect system reliability. In designing
reliability into the recoilless nfle weapon
systems, the designer muat remember that
reliability must be considered in the practical
context of the equipment being designed, i.e..
in the actual environmental conditions to
which the weapon system will be exposed-in¬
cluding operation, storage, and transporta¬
tion rather than in any abstract or theoreti¬
cal sense.
Considering these primary and secondary
purposes of the recoilless weapon system, it is
readily seen that reliability is an important
factor, in the design of the system, and it is
the design engineer who is in the position to
make the necessary contributions for a
reliable system, i.e., reliability must be
designed into the equipment; it cannot be
built in at a later stage in the system
development. The importance of reliability is
further emphasized by the fact that in
producing a reliable equipment, the manufac¬
turer will have solved simultaneously many
maintainability problems. Equipment that is
100 percent reliable for its intended useful
life requires no corrective maintenance.
Because reliability depends to a large extent
on both proper manufacturing procedures and
quality control tests, the design engineer is
again in the position to make major
contributions to the system reliability
through proper choice of fabrication, assem¬
bly, and inspection techniques. Although the
definition of reliability is straightforward, the
application of reliability to the weapon has
become an ever increasing problem, to the
extent that reliability engineering has become
a specialized field in its own right.
If the basic definition of reliability is the
probability of successful operation, the
reliability of the total system becomes the
product of the probabilities of its individual
parts to operate successfully (provided they
are statistically independent). It follows from
this that the more complex a system, the
greater the chance of failure. By using the
least number of subsystems and components
necessary to accomplish the system function,
i.e., by making the system as simple as
performance requirements permit, it is possi¬
ble to increase the reliability of the weapon
system. One exception to the general rule of
simplicity just described is caused by the use
of redundant (parallel 1 or back-up) subsys¬
tems.
Redundancy -providing back-up or alter¬
nate subsystems-provides a direct counter to
the previously defined product rule of
reliability because the higher the number of
alternate subsystems, the greater the prob¬
ability that one ot them-and the efore the
system as a whole-will operate satisfactorily.
At first consideration, the incorporation ot
redundant subsystems may seem incompatible
with the ideals of light-weight and compact
construction. However, with recent advances
in solid-state electronics, gyroscope tech¬
nology, hydraulics and other fields, light and
compact components for alternate subsystems
can be added without substantial increase in
total system weight and envelope. The criteria
for determining whether redundancy should
be designed into the system arc the resulting
9-43
Preceding page blank
AMCP 70*238
increase in reliability; increase in weight, size,
and complexity caused by addition of the
backup system; and the extent to which the
added component, subsystem, or system is
critical to successful operation.
Another method of increasing system
reliability is through the use of standard
components and techniques, the reliability of
which has been proven in many applications
and types of operations. If a system is
“designed” (implying that a new product is
being developed) with relatively untried
components and subsystems, the product rule
of reliability will generate lower system
reliability due to the lower reliability figures
of the individual components. Also, when
using new components, relatively little is
known about their behavior in a specific
environmental condition or when two or
more new components are combined or used
in an interacting application. The specific
conditions under which a subsystem is to
operate can markedly influence its reliability.
Standard components or parts are defined
generally as those which are commercially
available, i e., components which do not have
to be specially designed for a particular
application, and which conform to a Military,
Federal, c- industrial Specification or Stan¬
dard. Reliability history for these standard
parts is available from various military,
federal, commercial, and industrial trade
association sources, whereas, the necessary
history for a new system or component can
be obtained only through the performance of
accelerated life and environmental tests.
Other factors that contribute to the
reliability of the system as much as those
already discussed are the training of produc¬
tion workers to reduce human error, use of
proper organization to ensure exchange of
information within the design and production
facilities, and adequate testing. While these
areas are not primarily the concern of the
design engineer, he is often the first to be
iware of any problems related to them ariw is
often responsible for assisting in the required
change in manufacturing or quality assurance
procedures. Reliability also can be increased
during detail design phases by proper
selection of materials, protective fimshes, and
types of lubrication. These are discussed in
pars. 9-11 and 9-12.
9-11 MATERIALS
In the selection of materials for a recoilless
weapon system, several factors influence the
system reliability, namely
1. Stress
2. Impact
3. Friction, wear, and abrasion
4. Corrosion resistance
5. Effects o. high and low temperatures
6. Weight (portability).
In order to select the proper materials, the
designer must have complete knowledge of
the magnitude and nature of the operational
loads, and the resulting stresses to which the
weapon system will be subjected. Once the
stresses have been determined for the various
elements of the system, it is usually possible
to select the materials that provide the
necessary ultimate and fatigue strengths in
order to ensure that the components will not
kail under the action 'T prescribed loadir.g
conditions.
Impact is the second factor to be
considered in the selection of materials and is
one that influences the strength of the
material in bending, torsion, compression,
tension, and shear. Because a recoilless rifle is
subjected to impact loads during transporta¬
tion over various types of terrain and combat
conditions, it is necessary for the designer to
select materials that will provide the necessary
energy absorbing properties. Since it is
9-44
AMCP 706-238
difficult to ascertain the magnitude of impact
loads, the designer often will include requisite
impact loading factors in stress calculations to
ensure that the various components will be
designed adequately.
Because of the flow of hot propellant gases
in recoilless rifle operations, it is important to
select materials that do not exhibit high wear
rates and friction at the elevated operation
temperatures. These factors definitely are to
be considered in the design of breech, nozzle,
and gun tube since continued nozzle erosion
or gun tube wear immediately would reduce
the operational reliability of the system.
Recoilless rifle weapon systems are sub¬
jected to extreme variation of environmental
conditions, and, in order to prevent deteriora¬
tion of any component under these condi¬
tions, it is imperative to specify corrosion
resistant and fungous non-nutrient materials
in the design of all parts. Further, it is
important to avoid the use of dissimilar
metals in direct contact, since moisture can
promote galvanic action resulting in serious
corrosion. If it is necessary to use two
dissimilar metals in contact with each other,
plastic coatings may be used to provide
electrical insulation, but specification of any
electroplating, hot dip, or molten metal spray
should be made with caution since they might
be introducing new sources of galvanic action.
The mechanical behavior of materials under
conditions of high and low temperature is one
of the most important factors to be
considered in the selection of materials for
the weapon system. These conditions can
cause significant changes in such material
properties as creep, fatigue, tensile strength,
ductility, and cause oxidation, crack forma¬
tion, and surface deterioration of the
material. In general, strength properties
decrease and wear rates increase with
increasing temperature. Because of the ther¬
mal gradients that exist within the weapon
system during the course of operation, it is
also necessary to consider the effects of
thermal stresses induces and avoid the use of
dissimilar materials having different coeffi¬
cients of expansion. The use of such dissimilar
metals in contact could cause unequal
expansion and set up sufficiently high stresses
resulting in failure.
In the consideration of all the factors
influencing the selection of materials, it is
often necessary to weigh the relative signifi¬
cance of each of the opposing or limiting
considerations over the other in order to
achieve an optimum trade-off. For example, a
design problem involving strength versus
corrosion resistance may lead to the following
considerations:
1. Pure aluminum will meet corrosion
requirements for a particular component but
does ir>t meet strength requirements.
2. High strength aluminum alloys meet
strength requirements but are not as corrosion
resistant as pure metal.
3. Certain alloy steals have good strength
and corrosion resistant properties but will
introduce additional weight.
4. A combination of steel and aluminum
alloy parts may meet strength and corrosion
resistance requirements, but may create
problems of galvanic attack between the two
metals.
In order to make the proper choice from
among the given alternative compromises, the
designer must be able to make a quantitative
comparison of these alternatives with respect
to reliability and, in times of war, availability.
9-12 ENVIRONMENTAL DETERIORA¬
TION
There are three basic approaches to
designing for the adverse effects of environ¬
mental conditions to which weapon systems
are exposed. The first of these basic
approaches is to employ effective protective
9-45
AMCP 706-238
devices while using conventional components
and materials in the equipment itself. In this
approach, the component would be protected
against climatic extremes by use of shielding,
insulating, and cooling. Or, in the event of a
dynamic environment generating an impulsive
force, the equipment mignt employ some
type of shock absorbing mount or support.
While making use of conventional equipment,
this fust approach does not preclude the use
of any advanced technique for protecting the
equipment.
The second basic approach involves extra¬
polating known data on material properties,
and uses known concepts and design tech¬
niques to develop components that will
withstand specific environmental conditions.
The third basic approach, the newest of the
three, attempts to develop new methods of
designing materiel (especially electronic com¬
ponents) and new devices based on entirely
new design concepts.
Recoilless weapon systems are transported
through and used over the entire spectrum of
environmental factors. The process of protect¬
ing materiel against the adveise effects of
moisture (including salt u; ter), dust, sand,
snow, fungus, etc., is called weatherproof ng
and is accomplished in a number of ways.
These methods include the use of corrc 1 on-
resistant paints and finishes, sealing, potting,
wholly or partly enclosing, and the proper
selection of materials an \ components which
are chosen or adapted to fit the specific needs
of the design.
The various weatherproofing techniques
employ:
1. Mechanical finishing for smooth, polish¬
ed surfaces
2. Hot dip, electroplating, and molten
metal spray processes using copper, nickel,
chromium, tin, cadmium, zinc and lead
plating, and aluminum and anodic coatings
3. Case hardening processes
4. Phosphate and black oxide chemical
coatings
5. Numerous organic paints, including top
coats and primers.
The processes furnishing the best corrosion
protection involve the use of an inorganic
surface treatment (chemical or electrochemi¬
cal) over which an organic finish (primer and
top coat) is applied. Often, some departure
from the optimum corrosion protection must
be made because of other required surface
properties For example, a working surface
cannot be painted, but can still be given a
considerable degree of corrosion protection
by a chemical treatment or by electroplating.
In order to protect completely specific
components of the weapon system against the
effects of moisture, dust, and fungous spores,
it is necessary to enclose and seal the
components. Optical and gyroscopic devices
and some electronic components require this
type of protection since direct exposure to
some of the adverse climatic effects can cause
the component to become defective or
inoperable. Even though a part or component
of equipment is enclosed for protection, it
may also be necessary to seal the enclosure in
order to protect the component against
“oreathing” “Breathing”, the inward or
outward movement of air from the enclosure
due to variations in ambient pressure, can
cause infiltration of moisture, dust, and
fungou< spores into the enclosure. By use of
good sealing techniques, breathing can be
prevented completely to produce a hermetic
seal, provided that the nature of the design is
compatible to permit sealing.
Sealing techniques include impregnating
the pores of the enclosure case, minimizing
the cover contact area and use of gasketing
materials and coatings, use of Orings and
othe: seal rings on routing and sliding shafts,
AMCP706-23P
?
1
and potting. Potting is a special form of
hermetic sealing in which the component to
be sealed is ccated with a special potting
compound that serves to insulate the
component against certain climatic and
electrical environments. Potting finds its
greatest use in the sealing of electric and
electronic components that d' - not require
replacement or maintenance and which would
not be damaged during the potting process.
9-47
AMCP70S-2M
SECTION IV
MAINTAINABILITY
9-13 BASIC PRINCIPLES
In designing for the maintainability of a
specific equipment, the design engineer must
abide by a series of basic principles that are
dictated by the Armed Services maintenance
organizational structure. The first basic
principle is that all preventive maintenance
should be capable of being performed by
unskilled personnel even under adverse
working conditions. Secondly, in the event of
equipment malfunction, the cause must be
traceable, with a minimum need for special
instrumentation, to a subassembly or compo¬
nent that can be replaced easily as a unit
under combat conditions; or the malfunction
should be capable of being temporarily
bypassed or corrected by rimple operations
using simple tools. The third principle dictates
that vital equipment should be so designed
that it can be repaired rapidly by replacement
of defective parts at the field-maintenance
level, for quick return to combat units. The
last principle states that complete disassembly
and overhaul of the equipment need be
performed only at widely separated occasions
in order to avoid both the frequent
long-distance shipments to remote depots and
the overtaxing of depot facilities.
Maintenance pertains to actions necessary
to maintain an item in serviceable condition
while maintainability is a performance charac¬
teristic expressed as a probability (see
MIL-STD-778 and MIlrSTI>721).
9-14 ACCESSIBILITY
The importance of accessibility in mainte¬
nance is reflected in the following facts:
1. Hard-to-reach components requiring
preventive maintenance or frequent replace¬
ment are more likely to be neglected
2. Inaccessible or not easily isolated
equipment that needs to be energized or
operating during servicing and maintenance,
exposes the maintenance personnel to possi¬
ble electrical shock, contact with moving
parts, and other hazard
3. Ease of access reduces the probability of
human error
4. Easily accessible parts and components
reduce the time during which maintenance
personnel and vulnerable parts of equipment
being serviced would have to be exposed to
undesirable environmental conditions, if
maintenance must be performed under
adverse conditions. The design engineer must
keep these facts in mind during the course of
the design process.
The problem of accessibility can be
properly analyzed by asking the following
series of questions:
1. What routine maintenance will be
required? In answering this question, the
designer has listed the components that are
used during preventive maintenance or which
require frequent replacement.
2. What trouble-shooting will be required
at the organizational maintenance level, i.e.,
under combat conditions to restore the
equipment rapidly to operating condition?
3. Which components, assemblies, and
subassemblies will be removed and replaced as
units at the organizational maintenance level
and which will be adjusted at this level?
4. What trouble-shooting, removal of parts,
and adjustment will occur at the field-mainte¬
nance level?
9-49
Preceding page blank
A11CP 709*231
In answering this series of questions, the
designer will have generated the data needed
to specify the location and type of mounting
for specific parts or components of equip¬
ment and controls; and the location and
geometry of necessary access openings and
the type of access opening covers; i.e., hinged,
screw-fastened* or quick opening.
For a complete treatment of designing for
accessibility in maintenance, the reader is
referred to Ref. 12.
9-15 STANDARDIZATION
As pointed out in par. 9-1C, the use of
nonstandard components may decrease equip¬
ment reliability. Accordingly, the use of
nonstandard components may also increase
the required maintenance. The reasons for a
possible increase in maintenance as cited in
Ref. 12 are:
1. Nonstandard parts usually are stored for
longer periods of time because of their low
demand, a factor that tends to bring about
deterioration.
2. The larger the number of different
components in the equipment, the more
complicated the task of maintenance person¬
nel to install, handle, and maintain the
equipment.
3. Small-quantity production of non¬
standard items is characterized by lack of
uniformity and makes replacement parts more
difficult to obtain.
Because cf the reliability and maintenance
problems encountered in the use of nonstan¬
dard components, the Department of Defense
Standardization Program (see Refs. 11 and
IS) was prepared to guide design engineers in
the application of standarization to ail stages
of design. This resulted in ease and rapidity of
replacing and interchanging ptu cs and compo¬
nents.
In addition to standardization, the designer
should aim at maximum interchangeability of
components and subassemblies, i.e., for
different parts of the same equipment, the
same components should be used wherever
possible even though the functions of
assemblies in which they are installed may be
quite different. Maximum use of interchange¬
able parts leads to (1) efficient, uniform
maintenance procedures; (2) fewer repair
parts to support th equipment at every
maintenance echelon; (3) fewer failures; and
(4) lower costs. This interchangeability
concept is can.ed out for all subassemblies
and cc nponents (especially at the organiza¬
tional level).
It is also important to design equipment
that can be maintained and overhauled with
as few tools as the end requirements permit,
and with standard tools wherever possible,
rhe use of standard tools is particularly
important in the maintenance to be per¬
formed at the organization ar.J field levels.
Even at the depot level, the use of elaborate
test set-ups, special jigs, and the like should be
avoided. Maintenance tools should be con¬
sidered as an integral part of ell design phases,
so that design concepts requiring heavy or
elaborate maintenance and test equipment
can be discarded easily in the design study.
Also, by making small changes in design, it is
often possible t eliminate unnecessary tools.
For example, it will probably be found that
reducing the variety of fasteners used on a
piece of equipment will also reduce the
number of hand tools required.
REFERENCES
1. Recoilless Weapons, Volumes I to VI,
Contract No. W-36-034-ORD-7652,
W-36-034-ORD-7708, Franklin Institute,
Laboratories ior Research and Develop-
9-50
AMCP 706-241
meat, Philadelphia, Pa., for Ordnance
Department, US Army, May 1948.
2. Rene R. Studler and W.J. Kroeger,
Battalion Anti-Tank Recoilless Rifles
Systems, Repon No. R1273, Pitman-
Dun;. Laboratories, Frankford Arsenal,
Philadelphia, Pa., July 1953.
3. Robert Markgraf, Repeating Recoilless
Rifles, Pitman-Dunn Laboratories, Frank¬
ford Arsenal, Philadelphia, Pa., March
1960.
4. TM 9-3062, Operation and Organization
Maintenance 57 mm Rifles M18, M18A1
and T15E16, Tripod Mount M1917A2
and Weapon 'Tripod Mount M74, Depart¬
ment of Army Technical Manual, June
1975.
5. Development of 105 mm Battalion Anti¬
tank Weapons and Interior Ballistics,
Final Report, Contract No. DA-11-022-
ORD-1157, Armour Research Founua-
tion of Illinois Institute of Technology,
Chicago, Ill., Decembci (955.
6. Interim Technical Report on the Devel¬
opment of the 90 mm, Rifle TI49,
Contract No. DA-19-020-ORD-40, pre¬
pared by Arthur D. Little, Inc., for
Frankford Arsenal, Cambridge, Mass.,
June 1, 1955.
7. Notes on Development Type Materiel;
Cutridge, Heat, 90 mm T249E6 for Use
in the 90 mm T219E4 Recoilless Rifle,
Platoon Anti-Tank System (PAT). Report
PE-6, Ordnance Project No. TA1-1461
Prepared at Frankford Arsenal under
direction of Ordnance Research and
Development Division, June 1958.
8. M. Cohen, Preliminary Design Study of a
Recoilless Weapon Solution for the
Missile A Requirement, Report No.
R-1488, Frankford Arsenal, Philadelphia,
Pa., January 1959.
9. Human Engineering at Frankford Arsenal,
Human Engineering Report No. 1,
MR-553, Pitman-Dunn Laboratories,
Frankford Arsenal, May 1953.
10. Human Engineering Aspects of Recoilless
Rifle Design: M18 and T66 Series,
Human Engineering Report No. 8,
R1295, Pitman-Dunn Laboratories,
F.ankford Arsenal, Philadelphia, Pa.,
November 1955.
11. AMCP 706-327, Engineering Design
Handbook, Fire Control Systems-Gen¬
eral.
12. AMCP 706-134, Engineering Design
Handbook, Maintainability Guide for
Design.
13. AMCP 706-108, Engineering Design
Handbook, Elements of Armament En¬
gineering, Part Three, Weapon Systems
and Components.
14. MIL-STD-1472B, Human Engineering De¬
sign Criteria for Military Systems, Equip¬
ment and Facilities, 31 Dec 74.
15. Defense Standardization Manual
4120.3-M, Standardization Policies, Pro¬
cedures and Instructions, Office of
Assistant Secretary of Defense (Installa¬
tions and Logistics), January 1972.
9-51
AMCP70S-238
CHAPTER 10
RIFLE AND RIFLE COMPONENTS
1042 LIST OF SYMBOLS
A b = bore area, in?
P f = propellant gas pressure, psi
R = radius of projectile, in.
T = rifling torque, in.-lb
a = angle of rifling twist, deg
p - polar radius of gyration of projectile,
in.
AMCP7M4M
SECTION I
OVERALL DESIGN CONSIDERATIONS
10-1 GENERAL
The junctions of the various rifle compo¬
nents are closely interdependent, and, as such,
any variances in the performance of a specific
component affect the overall performance
uniformity of the weapon. Variations in
performance also can be attributed to human
and environmental interfaces as well. Items
such as gunner instability, improper mainte¬
nance, or drastic changes in weather or
environmental conditions may affect the
system performance as nuch as deterioration
in performance of any individual component.
In Part Two, Theoretical Analysis, of this
handbook, the theoretical background was
given for both interior and exterior ballistics
of the recoilless ritle weapon system. With
this background, it is possible to see how
factors-su^h as lot-to _>t variations in the
compositions of igniter, propellant, and
primer compositions or the effects of
temperature and humidity-will cause varia¬
tions in the peak clumber pressure and thus,
variations in the projectile muzzle velocity.
The last four chapters of this handbook
describe the specific design con siderations of
the major components of the recoiiless rifle
weapon system. In these chapters, the effects
of phenomena such as erosion, propellant gas
leakage, projectile balloting, bias, and solid
propellant lo>s on performance uniformity are
described in detail. Upon reading the last four
chapters, it is possible to understand that the
weapon designer is faced with th* fairly
difficult task of selecting the best features of
various components, given certain economic
constraints and integrating them into a
weapon system that is insensitive to the
factors that affect performance uniformity.
10-2 HAMMER BLOW
Most recoiiless rifles use mechanical types
of ignition systems to detonate the cartridge
primer. Since the primer is designed to be
somewhat insensitixe to shock and rough
handling, it is necessary to transfer a
sufficient amount of energy to the firing pin
for detonation of the primer to occur. In
order for this operation to be reliably
performed over the intended service life of
the weapon, a fairly stiff hammer spring is
used to accelerate the hammer prior tc its
impacting the firing pin. In the 1 ?0 mm HAW
(heavy antitank weapon) weapon system, the
hammer spring transfers 371 in.-oz of energy
to the firing pin (Ref. i). For reliable
detonation of the primer to occur, it is
necessary to insure that the lubrication used
in the firing mechanism is not fouled by the
propellant gas or fragments from the cart¬
ridge, and is not sensitive to temperature
extremes.
10-3 FIRING PIN
Firing pins used to detonate percussion
primers are cylindrical in shape, having a
diameter of approximately 0.30 in. and a
length to diameter ratio of 3 to 1. The tip of
the firing pin, which stabs the primer, is
hemispherical in shape with a radius of about
0.044 in. The firing pin, depending upon the
type of hammer used, may be solid or have a
bote on the rear of the firing pin for insertion
10-3
Preceding page bleak
AMCP 736-238
of the hammer mechanism. To prevent
fouling of the ruing mechanism, an oversized
flange is placed on the forward circumference
of the firing pin. This prevents passage of any
products of the firing from passing by the pin
into the firing mechanism.
10-4 PRIMER
The primers used in recoilless rifle ammuni¬
tion are generally of the small arms, 0.30 or
0.50-cal, percussion type. For example, the
primer for the 120 mm Cartridge. HEAT.
XM419 uses a 0.50-cul 50M primer. The
percussion primer and u supplementary FFFC
black powder charge of approximately 10
grains arc assembled into a metal tube and
this assembly is force-fitted, cemented or
threaded into the base of the cartridge or
ignit .ube assembly. Upon detonation, the
flames of the primer and ignited FFFG
powder charge vent through a flash hole in
the base of the igniter tub? or projectile boom
to ignite the booster charge.
Since the primer functions as the link
between the energy source (firing pin) and the
ammunition firing, it becomes a significant
key to the successful weapon operation In
order to define specific input characteristics,
primers are qualified with respect to sensitiv¬
ity requirements that reflect the amount of
energy that the firing pin is required to
deliver. Primer manufacturers customarily
provide in their data sheets the 100 percent
“all-fire" level of the primer. This all-fire level
is the mean firing height // plus five standard
deviations o for a specific weight dropped to
test the primer or (If + 5o) times (drop
weight). Since this sensitivity data represents
optimum test conditions, it is desirable to
provide an added margin of input energy-
usually taken as being equal to the required
all-fire level. As a general rule, it is best for
me designer to select the least sensitive primer
available which is ;■■owu>ol ible with firing
mechanism and environmental requirements.
10-6 BOOSTER
The type of charge used to ignite the main
propellant charge is A1 black powder. A
general rule calls for between 100 and 200
grains of powder per pound of main
propellant charge. As an example, the 120
mm Cartridge, HEAT, XM419 for the HAW
weapon requires a 2175-grain charge of At
black powder tor igniting a main charge of
10.7 lb of MS Propellant. The booster charge
is loaded into a cylindrical, perforated igniter
tube positioned in the center of the cartridge
case. Upon ignition, the booster produces a
hot gas that is distributed uniformly to ignite
the main propellant charge positioned around
the exterior of the igniter tube. In fixed
fin-stabilized ammunition, the projectile
boom serves as the igniter tube, with the main
propellant charge loaded around the projectile
boom. Maximum loading density of the black
powder booster charge usually is considered
to be 0.8 g-cnf 3 .
10-6 PROPELLANT
Both single- and double-base propellants
have been used in recoilless rifle ammunition.
Single-base propellants, of which M10 Propel¬
lant is an example, are those in which the
principal active ingredient is nitrocellulose.
Double-base propellants are those containing
nit r ocellulosc and a liquid organic nitrate
(nitroglycerin) which gelatinizes the nitrocel¬
lulose. M2 and M5 Propellants are examples
of double-base propellants that were used
almost exclusively in the early stages of
recoilless rifle development. Except in tne
case of the DAVY CROCKETT weapon
system, double-base propellants have not been
used in the later recoilless weapons intended
for repeated firings because they tend to be
excessively erosive in the nozzle and bore
areas. Cooler burning Propellants such as M10
and T 28 (M26) were used in later recoilless
rifle systems such as 90 mm MAW, 106 mm
BAT, aJv, . 20 HAW.
104
AMC '06-238
Single- and double-base propellants have
been used in both granular and single- or
multiperforated forms. Perforated propellant
grains normally have a length-to-diameter
ratio of 5 to 1, with web size between 0.020
and 0.040 in. as required to meet specific
performance requirements. Propellant loading
densities vary between 0.4 and 0.6 g-cm' 1 . As
described fully in Chapters S and 11* the
required chamber pressure is controlled by
the ballistician's choice of propellant compo¬
sition, granulation, and web. Changes in the
burning characterises can be accomplished
by inhibiting certain surfaces of the propel¬
lant grain from burning. For example, a
perforated grain may have its exterior surface
coated with an inhibitor material that allows
the grain to bum only on the surface of the
perforations.
10-7 CARTRIDGE CASE
In reccilless rifle weapon systems, it is
necessary to allow for the venting of the
propellant gases into the chamber. To meet
the requirements of the different types of
chamber-nozzle configurations, several types
of cartridge cases have been used. Tire
majority of recoilless rifles use perforated
steel cartridge cases with a line* inside to
cover the perforation and with provisions to
crimp the case to the projectile to fix the
ammunition. The perforated case allows the
propellant tc vent radially into the chamber
and rearward through the nozzle. In most
cases, cost limitations have required that
sidewalls of the perforated case be fabricated
from rolled, perforated sheets even though
the use of seamless tubing is much more
desirable for strength reasons. The cartridge
case base is then welded or brazed into the
cartridge case. Rupture or excessive case
deformation, as a result of firing, will prevent
extraction of the case. To meet the strength
and deformation requirements, the design
results in a case with rather unfavorable
weight characteristics.
The liner, which may be a Mylar film, serves
to confine the propellant in the case and must
withstand environmental and handling condi¬
tions while providing a moisture sea).
Crimping the cartridge to the projectile
serves a dual purpose in recoilless rifle
application: first, to “fix‘’ the ammunition
and achieve uniform alignment: and, second,
to provide shot start, i.e., the force required
to initiate projectile motion. Projectiles used
in recoilless rifles usually have either pre-en-
graved or essentially no engraving bands and
as a result shot start cannot occur while the
band is being engraved, as is the case with
artillery ammunition. Shot-start is a desirable
feature in that it contributes to performance
uniformity of the weapon. Too high a level of
shot-start will produce excessive initial for¬
ward recoil while too low a level will produce
undesirable muzzle velocity variations.
10-8 PROJECTILE
Additional design aspects must be evalu¬
ated after the projectile envelope has been
established. A rotating band located on the
projectile outer diameter is required in order
to center the orojectile in the bore, impart
spin, and to provide obturation for fin-stabi¬
lized projectiles. For spinning projectiles with
pre-engraved rotating bands, an additional
obturator consisting of a rubber or plastic ring
is needed. The rotating band may be an
integral part of the projectile or it may be a
separate copper material band swaged to the
projectile. Plastic bands aie used with
fin-stabilized projectiles, and they are ce¬
mented or ejection molded into place. The
height of the band is such that a minimum
clearance exists between the band and bore
rifling so as to minimize propellant gas
leakage. The band height must take into
account the degree of strain compensation
present in the tube, see pars. 10-14 and 10-28.
Another design consideration is to provide
AMCP706-238
a circumferential groove in the projectile
where the cartridge case is crimped in order to
obtain a fixed round of ammunition. Design
of the groove will depend upon the amount of
shot-start force desired for separation of the
projectile from the cartridge case. Two widely
spaced bourrelets (bore rifling surfaces) are
included on the fore and aft outer diameters
of the projectile, in order to eliminate in-bore
yaw (balloting) of the projectile.
For fin-stabilized projectiles, design studies
will consider several possible choices, fixed
versus semifixed and boattailed versus boom¬
tailed fins. Semifixed fins are generally used
on boattailed projectiles whereas fixed fins
are attached to boom type projectiles.
M 9 BREECH-CARTRIDGE RELATION
The interrelationships between the breech
and the cartridge, of interest to the designer,
include:
1. Installation of the round of ammunition
2. Initiation of the round
3. Retention of the case during firing
4. Proper flow configuration for recoil
compensating gases
5. Adequate sealing
6. Extraction of the case after firing.
The distance between the breechblock to
the seating point of the cartridge, headspace,
is of mqjor importance to the designer as it is
one of the factors determining firing pin
travel. Adequate sealing is essential to prevent
propellant gas from entering the firing and
breech mechanisms and causing erosive
damage and fouling of the components.
Proper clearance between the opening provid¬
ed and cartridge must exist in order to assure
effective chambering of the round.
106
10-10 CHAMBER-CARTRIDGE RELATION
Gas flow from the cartridge to nozzle is a
major factor influencing the chamber-cart-
ridge relations. Thus, in an annular nozzle
recoilless rifle sufficient clearance must be
provided between the cartridge and the
chamber wall so as not to restrict the gas flow
to the nozzle. In a central r.ozzle system, the
annular space between the cartridge and
chamber is not required since the case is
tangible. Upon firing, the case ruptures and is
ejected through the nozzle. However, it is
often necessary to extend the chamber length
to reduce solid propellant loss ir this central
nozzle configuration. The ladney-shaped
nozzle, by minimizing propellant loss, provid¬
es the configuration with the minimum
chamber volume and offers a resulting weight
savings.
Radial clearance between the projectile tail
fins and the cartridge case and the chamber
must be provided to prevent scoring the case
and/or the chamber.
10-11 TUBE-CHAMBER RELATION
Recoilless rifle ammunition using a perfo¬
rated cartridge case requires support by the
gun tube at the cartridge case mouth in rifles
with larger than bore sized chambers. The
mouth of the case is designed to minimize
deformation which would prevent easy
extraction of the case after firing.
The bourrelet of the projectile is designed
to give a minimum clearance between the
rifling lands and bourrelet diameters. Present
practice, as described in par. 11-9, requires a
clearance of 0.002 in. plus 0.001 times the
weapon caliber in inches. The clearance
between the rotating band and the rifling is
held to a minimum in order to minimize
propellant gas leakage past the projectile.
10-12 CHAMBER
The chamber configuration depends to a
mmaaaBBSBUk
asssggBBaaS
AMCP 706-238
great extent upon both the desired interior
ballistics and the type of nozzle-breech. Ihe
chamber volume can he calculated, as
described in Chapter 5, or- -Jie basis of a
desired muzzle energy, pro'- ucharge, and
peak chamber pressure. < i/ndri :al and
slightly forward sloping configurations have
been the most widely used of the chamber
contours, both having the advantage of
creating a fairly even pressure distribution
along the length of the cartridge case. In an
attempt to improve combustion efficiency
and to reduce the loss of solid propellant
grains, rearward sloping and baffled chamber
contours have been studied. However, these
contours led to unfavorable pressure distribu¬
tions across the cartridge case due to low
pressure areas outside of the cartridge caused
by high velocity gas flows at the narrowing
chambei sections. These pressure differentials
were severe enough to cause cartridge case
failure, and these contours fumrhed no
better performance than the cylindrical
contour.
10-13 NOZZLES
The nozzle is one of the most important
areas of design consideration since, through
the careful selection of tne nozzle parameters,
the weapon system is made recoilless. The
specific nozzle chaiacteristics to be deter¬
mined are the throat area, entrance area,
entrance radius, exit area, included angle, and
throat contour. The nozzle throat area is the
main determining factor in the control of
recoil since this area limits the rate at vhieh
the propellant gases may be discharged.
However, the other nozzle characteristics also
play a rol- in control of recoil since they
determine the flow characteristics of (he
discharging gases.
A secondary role that the nozzle plays is as
a torque compensator. Because the projectile
is given a designated spin as it travels through
the tube, it imparts a torque to the rifle. By
properly deflecting the discharging gases in
certain types of nozzles, it is possible to apply
a counter-twist to the rifle and thus neutralize
any torque imparted to the weapon by the
projectile. Section II of this chapter describes
these nozzle design considerations in more
detail and describes the enect that the various
nozzle parameters have on the control of
recoil.
10-14 TUBE
The length of the gun tube is limited by the
allowable weapon weight. Increased projectile
travel would permit a decrease in chamber
pressure but would caue an increase in
weapon weight. The length of the gun tube
then becomes a compromise between weight
and ballistic performance.
Because recoilless rifles are designed with
thin walls, the tube dilates greatly during
firing due to the propellant gas pressure and
thermal expansion. In highly stressed rifles,
this dilation becomes large enough to cause
excessive clearance between the projectile and
bore surf~.ee, with the result that ba'loting
(in-bore yaw) occurs as the projectile tra. /els
through the tube. To compensate for this
phenomenon, the principle of strain compen¬
sation is incorporated. A: initial interference
between the projectile and bore is provided so
that the clearance achieved during dilation of
the bore upon firing results in the normal
clearance. Because of the expansion of the
bore during firing, most accessories are
mounted on the tube by the use of thin metal
bands.
The forcing-cone region of the tube is the
interior tapered portion between the chamber
and bore, including the origin of the lands.
The forcing cone allows the rotating band of
the projectile to be gradually engaged by the
rifling and aids ir. centering the projectile
within the bore. The rifling may be either
uniform or increasing in twist. In general, the
use of increasing-twist rifling causes less wear
on the rifling grooves and lands, and reduces
10-7
FH".P
AMCP 706-238
the dangei of stripping the rotating band from
the projectile. However, the development of
the modem progressive burning powders
permits attainment of the desired muzzle
velocity with lower maximum pressures so
that the use of costly increasing-twist rifling is
not warranted.
10-15 SUMMARY
1 in the preceding paragraphs of this chapter,
a large number of factors which can affect
weapon performance are described Many
other not so obvious factors will lower
!
i
(
i
i
i
i
i
performance the same way. For example, the
booster charge in the igniter tube or tail-boom
cavity is often surrounded by a primer foil
liner or a nitrocellulose or cardboard capsule
which is then sealed for moisture-proof
protection. While not immediately obvious, it
was found that certain types of lacquer
coatings will inhibit the burning of the
booster charge, causing improper ignition of
the main propellant charge and resulting in
reduced muzzle mergy. Another considera¬
tion in the booster design is the combustibil¬
ity of the booster inclosure. The nitrocellu¬
lose capsule provides slightly improved
performance at lower temperatures.
10-8
AMCP 708-238
SECTION II
NOZZLE
10-16 GENERAL
Recoil'ess rifles discharge about 90 percent
of the propellant gases through the breech in
a rearward direction in order to balance the
momentum of the forward moving projectile.
Since the mass of the rearward moving
propellant gases is small compared to the mass
of the projectile, a converging-diverging
nozzle is used to give the propellant gases the
high velocity necessary to make the momen¬
tum of the propellant gases equal to and
opposite in direction to the momentum of the
projectile.
The design of a recoilless rifle nozzle is
based on four interrelated characteristics: (1)
the ratio of the rifle bore area to nozzle
throat area, (2) the ratio of nozzle approach
area to throat area, (3) the divergence angle of
the nozzle cone, and (4) the ratio of the
nozzle exit area to throat area (the expansion
ratio of the nozzle cone). The criteria used to
determine these characteristics are based on
rocket theory, with final nozzle dimensions
for a specific weapon design made on the
basis of empirical' data obtained from
balancing experiments. Empirical data have
shown that the ratio of rifle bore area to
nozzle throat area is approximately 1.4S,
provided
1. The nozzle approach area to throat area
ratio is greater than or equal to 1.70
2. The divergence angle (included angle) is
less than 1S deg
3. The expansion ratio of the nozzle cone
is approximately 2.C.
Information on nozzle performance sug¬
gests that the elimination of recoil is feasible
only over a limited range of the bore to throat
area ratio. In general, a decrease in the bore to
throat area ratio brings about a decrease in
the rearward recoil, as would be expected.
The point of zero recoil seems to occur at a
bore-to-throat area ratio of 1.45, however,
this value may not always be practical because
the redaction in recoil obtained by enlarging
the nozzle throat area will be accompanied by
a decrease in muzzle velocity.
In the design of converging-diverging
nozzles, it is necessary to avoid both the
inclusion of any discontinuities in the contour
of the nozzle and the use of largely divergent
angles. Both of these design characteristics
cause adverse pressure gradients in the
propellant gas flow and result in a decrease in
the rearward momentum of the gases being
exhausted. Since the r ecoil of the rifle is
directly dependent upon u.v momentum of
the exhaust gases balancing the projectile
momentum, it follows that retarding the gas
flow wil 1 result in increased rearward recoil.
10-17 NOZZLE EROSION
Nozzle erosion is the wearing away of the
nozzle inside surface caused by the impinge¬
ment of high velocity gas. Nozzle erosion in
recoilless rifles affects the overall rifle
performance, since the cross-sectional areas of
the nozzle are one of the limiting factors
controlling the velocity of the propellant gas
discharged. In many of the recoilless rifle
systems, the erosion problem was serious
enough to cause significant de: ign difficulties
and serious field maintenance problems. As a
10-9
AMCP 755-238
result, studies were commissioned during
various recoilless rifle programs to determine
how the erosion phenomenon could be
minimized.
One aspect studied was the effect of the
nozzle contour on the erosion process. Tire
general rule in nozzle design for minimizing
ercsion is that the nozzle contour allow the
propellant gases to follow natural lines of
flow while avoiding discontinuities in the
direction of flow. Some of the research done
on the problem was simply performed by
allowing the gas flow to shape its own best
nozzle shape. Examination of sectioned
nozzles, eroded from an initially square
entrance, showed, in most cases, a well
defined circular entrance with the radius
equal to the diameter of the throat (Ref. 2).
More recent results have indicated that the
divergent angle has a negligible effect on the
erosion process and that elongating the basic
circular shape (as in a kidney-shaped nozzle)
also has no appreciable effect on the rate of
nozzle erosion (Ref. 3).
In addition to nozzle contour, several other
factors also may affect the erosion process.
Studies have indicated that surface melting is
the dominant mode of erosion in the nozzles
of recoilless rifles. As such, the choice of
nozzle material, propellant composition, and
the rifle rate of Are have a definite effect on
the erosion rate. Nozzle erosion tests with
various pure metals and alloys, both ferrous
and nonferrous, used as the nozzle material
have indicated that the best metals frem the
standpoint of erosion resistance were pure
metals such &« molybdenum, tungsten, chro¬
mium, beryllium, and tantalum. C. dinary
cold-rolled steels have shown the most
satisfactory performance of all the steels b’ t
are much less resista.t to erosion than the
pure metals -ited.
Another factor influencing the erorion
proewss is the isochoric (constant volume)
flame temperature of the propellant used in
the cartridge. In general, it has been shown
that erosion is reduced through use of a
propellant with a lower flame temperature.
However, for a specific nozzle material, there
is an optimum propellant for which little or
no improvement will result through the use of
cooler burning propellants.
The rate of erosion also is affected by the
rate at which the weapon is fired. Rapid firing
has an adverse effect on erosion resistance ftr
two reasons: the time required for the nozzle
material to reach its melting point is reduced,
and the amount of heat capable of being
conducted away from the nozzle after melting
starts is reduced.
In designing for erosion resistance it
probably will be necessary to make certain
compromises in the nozzle design. In general,
it will not be feasible to use the coolest
burning propellant or the most ercsion-resis-
tant material because performance, strength,
and cost requirements may dictate the use of
other propellant and nozzle materials. In most
cases, the nozzle purposely is designed slightly
undersized so that the rifle has some rearward
recoil for the initial firings. As the nozzle
erodes, the point of zero recoil eventually is
reached. Further erosion of the nozzle under
more firings eventually causes a forward recoil
condition to be reached. However, the life of
the rifle nozzle has been increased by as much
as SO percent by not sizing the nozzle to given
an initial zero recoil.
10-18 VARIOUS TYPES OF NOZZLES
10-18.1 CENTRAL NOZZLE
The central nozzle, as shown in Fig. 10-1,
can be considered an optimum design in the
sense that it is the simplest and lightest of the
nozzle designs. The central nozzle concept
generally requires the use of a blow-out plug
or valve at the rear of the cartridge case. The
blow-out plug assures retention of the
propellant gases in the cartridge case and
10-10
AMCP 706-238
chamber until sufficient combustion has
taken place. The plug then ruptures rnd
allows the propellant gases to escape through
the nozzle. While the central nozzle offers
definite advantages in weight and simplicity,
there are a number of disadvantages that must
be considered.
One problem encountered when a spin-sta¬
bilized projectile is Area in ?. recoilless rifle
with a central nozzle occurs because the
central nozzle has complete axial symmetry
about the rifle axis. As a result of this
symmetry, no means are available for
applying a counter torque by controlled gas
discharge on the rifle to compensate for
reaction to the spin of the projectile.
However, vaues were placed in the central
nozzle of early mortar type guns. Another
problem associated with the use of the central
nozzle is that it tends to lose more unbumed
propellant through the nozzle than other
types of nozzles and, therefore, yields less
uniform ballistic performance. Further nore,
some difficulty is encountered in providing
for both a readily accessible firing mecuanism
and removal of the expended cartridge case.
Accessibility for firing often is achieved by
either attaching a firing line, commonly called
a pigtail, to an electrically initiated primer or
by housing a centrally located firing device in
the nozzle, such as found in the bar breech
type nozzle described in par. 10-18.2. The
problem of extracting the spent cartridge case
is eliminated through the use of an expend¬
able or frangible cartridge case.
General design practice with early super¬
sonic (converging-diverging) nozzles for rock¬
ets required a nozzle contour with a
well-rounded entrance section joined to
truncated cones with a 14-deg total divergent
angle. It was not until theoretical concepts
developed in the fields of aerodynamics and
jet propulsion were applied to the central
nozzle recoilless rifle that it was found that an
angle of 40 deg was the largest divergent angle
that could be used. However, exp'rimental
work witn dive,gent angles has shown that
angles as large as 45 deg may be employed
with some sacrifice in efficiency if a
significant weight savings was to be gained
(Ref. 3).
10-18.2 CENTRAL NOZZLE WITH BAR
As stated in par. 10-18.1, the bar breech
type nozzle is a special configuration of the
central orifice nozzle. The bar breech derives
its name from the bar which is centered across
the nozzle exit and houses the firing
mechanism, as shown schematically in Fig.
10-2. The nozzle throat and exit areas are
Figure 10-2. Central Nozzle With Bar
10-12
AMC? 706-238
adjusted to compensate for the bar interfer¬
ence. The advantages are much the same as
outlined for the central orifice nozzle in par.
10-38.1, i.e., simplicity and lightness in
weight. The disadvantages in using th- breech
bar type central nozzle are fouling of the
firing mechanism by the propellant gases and
the erosive effects of the propellant gases on
the breech bar. Another undesirable feature is
that the rifle loader may have to pass his hand
in back of the nozzle after the round has been
chambered in order to close the breech bar.
This action may be avoided by the inclusion
of extended handles attached to the breech
bar; however, in doing so, weight is added to
the rifle.
10-18.3 CENTRAL EXPANDING NOZZLE
As stated in par. 10*18.1, one of the
disadvantages in using the central nozzle
design concept is the problem associated with
chambering and extracting the round of
ammunition. As in all recoilless rifle nozzle
designs, it is necessary that the nozzle throat
area be less than die bore area in order to
achieve the recoilless condition. Therefore,
for the rifle to be breech-loaded, provisions
must be made for either enlarging the nozzle
or for using a "breech door” or “breech bar”
which gives the necessary opening for
chambering the round ; nd subsequently
reducing the nozzle area.
The 90 mm Rifle, T234, employs a nozzle
design that can be considered novel from the
standpoints of design and of operation. In
order to simplify and minimize chambering
and extracting operations, the T234 Rifle
employed a central expanding nozzle. The
T234 Rifle nozzle is, itself, the breech and is
segmented into eight dose fitting sections
which are spring-loaded to the closed
condition as shown in Fig. 10-3 (kef. 4j. The
insertion of a round of ammunition causes the
segment to move forward and expand
radially outward to permit the larger diameter
projectile to be fully inserted. What makes the
use of the central expanding nozzle possible,
however, is the use of a frangible cartridge
case. Instead of the conventional perforated
metal cartridge case, a thin “powder enve¬
lope” is used. This envelope is destroyed
during the ballistic cyde, thereby eliminating
the need for spent-case extraction.
10-184 MULTIPLE NOZZLE AND FRONT
ORIFICE
The multiple nozzle, front orifice type,
recoilless rifle has the distinctive feature, as
shown schematically in Fig. 104, that tht
projectile covers the nozzle ports at the
beginning of the ballistic cyde. There are
several advantages of this design as compared
with rear orifice designs. These advantages
are, as described in Ref. S:
1. A nonperforated cartridge case may be
used, thus contributing to manufacturing cost
savings.
2. Since the propellant charge is confined
to the closed ballistic system at the beginning
of the ballistic cycle, better ignition character¬
istics of the propellant result.
3. The loss of unburned propellant
through the nozzle is negligible as a result of
the closed system burning at the beginning of
the ballistic cycle.
4. Nozzles of large expansion ratio can be
used without lengthening the rifle.
5. The closed-chamber system will permit
design variations to include automatic loading
mechanisms end venting of the propellant
gases through exhaust ducts.
The disadvantages in using the front orifice
system ire:
10-13
AMCP706-23S
ffci.S-3 '.wi:
-,i;s
/vpu/ie /Os?. Central Expanding Nozzle
1. 4 slight increase in chamber weight.
2. An increased mount weight. Since there
is an initial recoil of the rifle until the nozzles
are uncovered, the rifle mount must be
constructed accordingly to withstand the
recoil forces
10-18.5 ANNULAR NOZZLE
As shown in Fig. 10-5, the annular nozzle
has the advantage that it is readily adaptable
to most chamber and breech construction.
With the annular orifice, the cartridge case is
supported adequately by a solid base in the
breechblock. Also, the firing mechanism is
housed within the breechblock, thus elimi¬
nating the need for any nonpermanenl type
of firing line or pigtail. The annular nozzle is
also advantageous in that, for the same degree
of expansion, the nozzle can be made shorter
than for a central type nozzle. The reason is
that the exit area at any distance from the
throat is larger in the annul a nozzle design.
Another advantage of the annular* nozzle is
revealed in the consideration of rifling torque
compensation. In the annular nozzle design, it
is possible to shape and cant the locking vanes
to the breechblock so that the resulting
rotational impulse of the deflected propellant:
gases escaping rearward balances the impulse
[f
AMCP 706-236
Figure 10-5. Annular Nozzle
due to the reaction of the nils to the
projectile spin.
A disadvantage of the annular nozzle is that
since the locking vanes secure the breech in its
closed position and deflect the discharging
gases for the purpose of torque neutralization,
they are subjected to the full erosive effect of
the gases. The locking vanes also bear the full
reaction load caused by the chamber pressure
acting on the cartridge case and must be
readily locked and unlocked during loading
and unloading.
10-18.6 INTERRUPTED ANNULAR NOZ¬
ZLE
In actual practice, all annular nozzle
designs are of the interrupted type as shown
in Fig. 106, since there must be some means
of supporting the breechblock in the locked
position. The interrupted annular nozzle
maintains the same advantages and disadvan¬
tages described in par. 10-18.5 for the annular
nozzle.
10-18.7 KIDNEY-SHAPED NOZZLE
A modification of the interrupted annular
Figure 10-6. Interrupted Annular Nozzle
AMCP70S-2M
nozzle ii the kidney-shaped nozzle that is
shown schematically in Fig. 10-7. It has the
advantage of complete symmetry while
providing the solid center for housing the
firing mechanism. The kidney-shaped nozz'e
easil y adapts to rifling torque compensation
by canting the nozzle sections or channels.
The disadvantages of this nozzle are its
complexity and extra weight, which make it
the most costly of all the different types of
nozzles to produce. It also has the tendency
to develop cracks in the webs between th '
orifices.
Experimental ballistic data (Ref. 3) also
have indicated that the kidney-shaped nozzle
is slightly less efficient than the central nozzle
design. In addition, the rocoil compensation is
Figure 10-7. Nozzle Wi\
more sensitive to changes in the approach area
in the kidney-shaped nozzle than in the
central nozzle. Lastly, the ballistic efficiency
of the kidney-shaped nozzle is slightly less
t han that for th* central nozzle. However,
because of the cartridge case support it
furnishes, the kidney-shaped nozzle has found
much greater use. Rifles that have used
kidney-shaped nozzles are:
1. 57 mm Rifle, Ml8
2. 7S mm Rifle, M20
3. 105 mm Rifle, M27
4. 106 mm Rifle, M40.
AMCP 70G-23V
SECTION III
BREECH
10-19 GENERAL
The hreech is the rear part of the recoil!css
rifle through which the projectile is loaded
into the chamber and through which the
rearward discharge of gases usually occurs.
The breechblock contains the firing pin, and
locates and holds the head of the cartridge
case. A breechblock operating mechanism
unlocks and withdraws the breechblock from
the breech, swings the block clear, and returns
it to the firing or closed position.
The breechblock components are a locking
device, a cartridge extractor and ejector,
safety devices, a percussion mechanism, and a
nozzle throat adjuster. The operation of the
breechblock consists of opening, extracting,
and ejecting the expended or fired cartridge
case from the chamber, chambering the new
round of ammunition, closing the breech¬
block, and firing the round.
10-20 CHARACTERISTICS
The type of t ech used will depend on
what the requirements are for supporting the
chambered round, the size of tire rifle, the
type of firing mechanism, and the type of
aumunition. With the use of a frangible
cartridge case envelope, it would be possible
to incorporate the unique expanding nozzle
type breech described in par. 10-22. In
another rifle, the strength requirements for
supporting the perforated metal cartridge case
may dictate the use of an interrupted lug or
interrupted-thread type breech to provide the
necessary strength.
limitations on the breech design. In general, a
shoulder-fired rifle design necessitates the use
of a lightweight breech mechanism. If the rifle
is a repeating type used in ar. enclosed
application, a front orifice rifle may be used
so that the breech design is concerned only
with the chambering and extracting of the
ammunition.
It is necessary to determine that the critical
parts of the breech and firing mechanisms
have been protected from the erosive and
fouling effects of leaking propellant gases. In
par. 10-21, the various means of sealing or
directing the flow of leaking propellant gases
are discussed more fully. It is also necessary
to insure that a good seal is obtained between
the breech and chamber if they are joined. A
leakage path between the chamber and breech
will be expanded rapidly due to erosive
effects and will result in a decrease in weapon
performance and an increase in safety
hazards.
In breech design, human engineering and
safety characteristics must be considered. The
breech mechanism should be easy and quick
to operate. If the breech is manually
operated, it must uot delay the rate of fire.
During continuous firing, the loader should
not become unduly fatigued.
For safety, several characteristics of the
breech design must be considered. Handles
mounted to the breechblock should be
located so that the loader does not have to
pass his hands or any po.'tion of his body
behind the nozzle while locking the breech. It
should be impossible for the firing mechanism
to operate until the breechblock is locked
10-19
Preceding pap blank
The size of ihe rifle also will place
AMO»M*28»
securely in place. No gas leakage paths should
be present in the breech-chamber interface.
10-21 SEALING PROPELLANT GASES
In the design of the breech mechanism of a
recoPJeaa rjflr. it might seem that since there
is such a large escape of gases through the
nozzles that there would be no concern about
gas leakage. However, gas leakage can be
extremely important because it can result in
the erosion of locking surfaces and other
important parts of the breech mechanism.
This easily can be seen in the original
blow-out disc type recoilless rifles designed by
the British and Germans in which leaking
propellant gases caused continuous malfunc¬
tions of the firing mechanisms as well as
causing considerable erosion damage.
In larger conventional artillery weapons,
the use of obturating pads, obturating rings,
and the expansion of the cartridge case by the
propellant gase. are employed to form a tight
seal against the walls of the chamber. In
recoilless rifles, this type of gas leakage
prevention cannot be used. With the use of
the perforated cartridge case, the gas pressure
inside and outside the cartridge case is
essentially the same, and the case cannot be
expected to expand in order to effect sealing.
To prevent the propellant gases from reaching
critical parts of the breech and firing
mechanisms of recoilless rifles, annular
grooves or leakage paths are designed to direct
the leaking propellant gases away from critical
areas by providing small expansion chambeis
in which the pressure of these leaking gases is
reduced.
10-22 BREECH TYPES
A variety of breech mechanisms and
breechblocks have been or are being used. The
most widely used breechblock types are the
interrupted lug and interrupted thread types.
However, all the breech types and mecha¬
nisms described in the remainder of this
paragraph have been designed, built and
tested fer use in recoilless rifle systems.
in the interrupted thread breech, the
breech recess and the breechblock are cut
with a series of stepped threads so that when
the breechblock is inserted and turned in the
breech recess, matching sections of threads
engage. The interrupted thread type of breech
gives a large threaded surface or holding area
which provides the necessary strength for
holding the cartridge case in position during
firing. The interrupted lug breech is similar to
the interrupted thread type ot breech except
that interrupted lugs replace the threads on
the breechblock and chamber.
In order to reduce the weight of the overall
weapon system, several recoilless rifle weapon
concepts have incorporated the cartridge case
as a portion of the chamber and breech
closure. This is achieved, however, by a
considerable penalty in ammunition cost and
weight- The 90 mm Rifle, T149 is an example
of such a weapon. To hold the cartridge case
in place, the breech mechanism is a rotating
cam ring as shown in Fig. 10-8, the cam ring
locks the round in place, cocks the firing
mechanism, and actuates the extractor. The
cartridge case base has two diametrically
opposed projecting lugs that, upon proper
chambering of the round, firmly seat in
corresponding sockets in the rear of the
chamber. Counterclockwise rotation of the
cam ring brings two locking lugs on tire ring
into register with the cartridge base projec¬
tions to secure the round.
In further efforts to reduce the weight of
the breech mechanism and also to eliminate
the manual operations for operation of the
breech, one of the initial designs of the 90
mm Rifle, T234, employed a central expand¬
ing nozzle, which is itself the breech,
segmented into eight close-fitting sections
that are spring-loaded to the closed position.
Insertion of the round of ammunition causes
the segments to expand radially outward to
10-20
I ' UHl" \lSft i fffir fn nilb*
njH ^.wwi ' - i f a y
AMCP 706*238
permit insertion of the larger diameter
projectile. The incorporation of such a breech
mechanism was made possible in this specific
cose because the cartridge case was of the
frangible type being destroyed during the
ballistic cycle and not requiring spent case
extraction.
Another type of breech mechanism studied
during the 90 mm Super-PAT (platoon
antitank) Program employed the idea of
inserting a cylindrical bar through the nozzle
of the weapon (Ref. 6). The purpose of the
bar was to provide a surface that the
propellant gases would strike in order to
establish a condition of no-recoil. Recoil
would be eliminated by varying the dimeter
and thus the area of the nozzle. This design
was intended to simplify the mechanics of
loading. Loading of the round would be
accomplished by first sliding the breech bar
out, inserting the loaded projectile into the
chamber, and then replacing the breech bar.
10-23 BREECH ACTUATOR
The breech actuator or operating handle
mechanism of the breech contains the
mechanisms for performing the locking,
unlocking, opening, and closing operations of
the breech. In most of the early design
recoilless rifles, the handles were connected
directly to the breechblock. Opening of the
breech consisted of first rotating tire breech
handle abort the bore axis until the
interrupted lugs or interrupted threads in the
breechblock were disengaged from their :
mating parts in the breech housing and then
swinging the breechblock outward about
some type of hinge. The 57 mm Rifle, Ml8,
and the 75 mm Rifle, M20, are examples of
rifles with this type of breech operating
handle. To close the breech, these operations
are performed in the reverse order. In many
r lilless rifle designs, these operations also
.--.use the cocking of the firing mechanism.
In later recoilless rifles, the breechblock
operating mechanisms were designed so that
the loader's hands would not have to pass
behind the nozzle, as is the case when the
breechblock handles am rigidly attached to
the breechblock. For example, the breech¬
block operating handles for the 105 mm
Rifle, M27, and 106 mm Rifle, M40, are
connected to the breechblock by a set of
cams and coupling rings which permits
rotation or translation and opening of the
breechblock without passing the handle
behind the breech. The handles are attached
at the side of the breech and are operated by
only a counterclockwise rotation at the end
of the handle above the real’ of the breech.
Operation of the breech actuator also
functions a cartridge case ejection mechanism.
The extractor usually consists of a finger type
projection that sits under the lip of the
cartridge case base. When the breech is
opened, the extractor hooks the cartridge case
base and slides the case partly out of the
breech so that the case can be removed easily
by the loading personnel.
10-22
AMCP 784-238
SECTION IV
CHAMBER
10-24 GENERAL
The section of the recoilless rifle which
houses the round of ammunition is called the
chamber. The chamber contains a rear
opening or breech mechanism thr ugh which
the projectile is loaded and a front-opening
leading into the bore of the tube. After
chambering of the round, the part of the
projectile forward of the rotating band rests
in the bore. The remainder of the round rests
in the chamber with the cartridge case base
supported at the breech and the cartridge case
mouth supported at the forcing cone.
The function of the chamber and its
relation to the cartridge case will depend
upon the type of nozzle-breech design used in
the rifle. If annular or kidney-shaped nozzles
are used with a perforated cartridge case, it is
necessary to provide a sufficient annular
volume around the cartridge case to allow for
the venting of the propellant gas and the
resulting flow of gases to the nozzle. Ii. the
central nozzle rifle, the large annular chamber
volume is not required. The contour of the
chamber is either cylindrical or forward
sloping, since both of these configurations
give a uniform pressure distribution across the
cartridge case.
10-25 SIGNIFICANCE OF CHAMBER VOL¬
UME
The chamber volume is a major characteris¬
tic in a recoillest. rifle design in that it is
directly lelated to the pressure level and
muzzle velocity and, correspondingly, to the
weapon weight and length. Chapter 5, ’nterior
Ballistics, contains detailed analysis relating to
the effect of chamber volume on performance
and design of the weapon. Ref. 5, Chapter 5,
indicates that the variation in peak pressure
and muzzle velocity are inversely proportional
to a change in chamber volume. Peak pressure
varies inversely less than twice the change in
chamber volume while the muzzle velocity
change is only about 0.2 times that of the
inverse in the chamber volume variation.
10-26 EJECTION OF PROPELLANT
In firing tests of the first recoilless rifle
designs, it was learned that the amount of
unbumt propellant grains ejected through the
nozzle was roughly inversely proportional to
the average distance the nropellant grains
must travel to be ejected. Thus the first
attempts at the prevention of propellant loss
investigated the concentration of the cartridge
case perforations at the mouth end of the case
so that, on the average, the propellant would
travel a greater distance before being ejected.
However, the resulting uneven pressure
distribution on the cartridge case led to many
cartridge case failures and the eventual
abandonment of this approach.
Experience with standardized US recoilless
rifles has shown that solid propellant loss can
be maintained below ten percent when a
kidney-shaped nozzle along with a perforated
cartridge case is incorporated in the weapon
design. The 106 mm Rifle, M40, has this type
of nozzle <n combination witn the recoil
compensating device described in - :r. 9-3.7 to
prevent solid propellant loss. The problem
with this type of nozzle arrangement is that a
10-23
nrr mssamsfayM
AMCC 706-238
large, annular area about the cartridge case
must be provided so that propellant gases
have an unrestricted path to the nozzle. The
larger annular area increases weapon weight.
Weapons can be designed employing a
central nozzle and a frangible cartridge case,
as in the 90 mm, M67 Recoilless Rifle. With
this combination, it is not necessary to
provide a large annular area about the case
;*ince the case ruptures and is blown out of
the gun upon firing. This type of recoilless
weapon offers apparent promise of lower
chamber weight than a rifle with a compara¬
ble kidney-shaped nozzle; but, the solid
propellant loss is on the order of 30 percent
of the total charge. Because of this factor, a
larger chamber volume is required, the weight
of which tends to counterbalance savings
achieved by the use of a central nozzle and
frangible case. It is, therefore, desirable to
reduce the propellant loss in the central
nozzle system to a level comparable with or
less than that of the kidney-shaped nozzle.
This not only will reduce the ammunition
weight but will further reduce the weapon
weight, since the chamber volume can be
decreased as the propellant loss is reduced.
In principle, achievement of simultaneous
nozzle-start and shot-start can provide for
leduction in solid propellant loss in a central
orifice recoilless rifle. Simultaneous nozzle-
start shot-start occurs when a means for
mutually sealing the nozzle and holding the
projectile is provided in the ammunition
design. Thus, when the holding member fails,
gas flow out of the nozzle and projectile
motion are initiated simultaneously. Further¬
more, the magnitude of the stait can be
controlled by adjusting the force required to
part the holding element. In the event
simultaneous start occurs at “all burnt”,
propellant loss must be zero due to the
corresponding closed-bomb burning resulting
from start occurring at “all burnt”.
Ideally, a central orifice recoilless rifle
could be designed to yield a zero propellant
loss by merely providing that the simultan¬
eous start occur at or slightly below “all
burnt”. However, the resulting weapon will be
longer and heavier than one designed for
zero-start, and the advantage of reducing
propellant loss to zero will be overshadowed
completely by the increase in weapon weight
and size.
10-24
m
5P!
w/m
r*mj*
AMCP 706-238
SECTION V
TUBE
10-27 GENERAL
The tube gives the projectile direction and
a rotating motion for the purpose of
aerodynamic stability. The tube may be a
separate member attached to the chamber or
may be integral with the chamber. In either
case, the tube will require the same material
properties as outlined for the chamber.
Since the tube represents the largest
member of the recoilless rifle, it is necessary
to assure that the tube weight is a minimum
while still maintaining the required strength.
The minimum p ossible wall thickness at the
point of maximum pressure must be deter¬
mined (Ref. 11).
Because of the geometry of the tube and its
accessibility, it would seem that the tube is an
ideal member for attaching the various
sighting and spotting accessories. In many of
the recoilless rifle applications, this is true.
However, in rifles in which the tube is made
from a high-strength steel stressed to its high
limit, the tube is highly strained during firing.
As a result, at a specific point, the tube
expands and contracts as the projectile passes
this point, causing difficulties in designing the
bands for mounting the various other
accessories to the tube. There are many
specific design considerations that need to be
made in the design of the tube, and these
factors are discussed in the remaining
paragraphs of this section.
10-28 DESIGN CONSIDERATIONS
In the design of the recoilless rifle tube,
one of the first factors considered involves the
principle of strain compensation. If a tube
uses either a high strength steel stressed to its
high limit or a material having a low modulus
of elasticity, the tube will be highly strained
during firing. The possibility arises that the
projectile could be completely disengaged
from the tube rifling. Use of the strain
compensation principle requires that the
projectile fit the tube during firing, rather
than prior to firing, as described in par. 10-14
and Ref. 7.
Rifling is the term given to describe the
helical grooved pattern cut in the bore
throughout tlu gun tube. The surfaces
between the grooves are called the lands.
Through interaction of the projectile rotating
band with the rifling, the rotation required
for flight stability is imparted to the
projectile. The twist of the rifling at any point
is the inclination of the groove to the element
of the bore through the point. The twist may
be uniform, increasing, or a combination of
the two, and is expressed in terms of the
number of calibers of length in which the
groove makes ne complete turn. The exact
shape of ?he rifling will depend on ballistic,
strength, and wear factors. The rifling twist
depends upon the desired rotational velocity
of the projectile at the muzzle.
One of the main purposes of recoilless rifle
development was to provide infantry with a
lightweight, armor defeating weapon. To meet
this requirement, it is necessary to use high
strength materials for the tube so that the
tube wall thickness and, thus, weight is
minimized.
Another important tube design considera-
10-25
AMCP 70S-238
tion is eccentricity. Since the recoiliess rifle
tube wall b designed to fce as thin as strength
requirements ahow, any deviation from the
tnir. circular path of the tube results in the
loss of metal thickness at the particular point
that the eccentricity occurs. Even if the tube
has an eccentricity o'' only several thou¬
sandths of an inch, an appreciable fraction of
the actual barrel strength would be lost.
Consequently, all tube designs require specifi¬
cation of the permissible eccentricity.
Another factor of the tube design requiring
consideration is the forcing cone and
bore sight grooves. The forcing cone is tl/e
interior tapered portion of the tub; between
the chamber and the bore, including the
origin of the lands. The forcing cone area
allows the pre-engraved rotating band of the
projectile to be engaged gradually by the
rifling and aids in centering the projectile
within the bore.
A small but important consideration
concerning the tube design is the incorpora¬
tion of boresight grooves on the muzzle.
The design of some recoilless rifles requires
the joining of the tube to the chamber. The
factors that will determine the method by
which these two rifle components are joined
are the chamber and tube materials, and their
wall thicknesses. Threading the tube into the
chamber will depend on the ability of these
components to resist distortion under applica¬
tion of installation torque. T t is essential to
provide sufficient torque to prevent the tube
from rotating loose during firing. If the tube
is to be brazed to the chamber, material
properties need to be considered. If they are
joined by brazing, it will not be necessary to
maintain quite as close manufacturing toler¬
ances since a gas tight seal between chamber
and the barrel will be formed.
10-29 OTHER SUBJECTS TO BE CONSID¬
ERED IN DESIGN
As the recoiliess rifle designer endeavors to
minimize the weapon weight, a problem arises
with the rifle overheating during sustained
rapid firing. In the recoilless rifle, this heating
is more rapid than in a conventional gun due
to the low heat capacity of the rifle tube. For
a given weapon-round combination, a specific
amount of heat is transferred tounit area of
the tube per round. The temperature rise in
any section of the rube is then inversely
proportional to the mass of that section.
Thus, any ’ttempt to reduce the tube weight
results in a higher temperature rise per round
nnd a faster approach to the temperature at
which the yield strength of tube material
rapidly decreases. Since the design streses
often approach the yield point of the tube at
norma 1 temperatures, it is possible for the
yield strength to be exceeded tmder condi¬
tions of sustained or rapid firing.
Cook-off is the deflagration or detonation
of a round of ammunition ouc to the
autoignition of either primer or propellant
and booster charges. In the case of the
hand-loaded recoilless rifle, the chance of
cook-off occurring is negligible since the rate
of fire is not high enough to cause a sufficient
increase in weapon temperatue and there is no
need to keep the round chambered for any
length of time. In automatic recoiliess rifles,
cook-off becomes more of a concern but is
still not likely to occur because the low heat
capacity of the gun tube will limit the number
of rounds that may be fired in a rapid burst.
The primary danger of cook-off of a
chambered round will be to the personnel and
equipment in the line of tire and behind the
weapon.
Ref. 8 gives an indication of the bore
temperature expected after firing a recoiliess
rifle. For a single firing of a 57 mm Rifle,
AMCP 706-23$
T170, it was found that the maximum
attained bore temperature occurred at the
origin of the rifling and avereged about
470°F. Along the barrel, the bore tempera¬
ture dropped in an almost linear manner,
reaching only 160°F at the muzzle. In tests of
the 106 mm BAT Weapon System, it was
found that the maximum rate of fire was
limited to two rapid bursts of four rounds
each before the upper temperature limit of
the rifle was reached (Ref. 9).
During projectile travel through the bore,
the gun tube rifling offers considerable
resistance to the projectile rotating band. This
resistance appears in the form of a radial force
acting on the rifling and is distributed
unife 'nly around the bore. These radial
forces or band pressures progress along the
tube with the projectile. Although the band
pressures may be large, the area of application
is local and small with a very short duration,
so that immediate damage is not always
apparent. However, repeated application of
such band pressures may ultimately damage
the bore. Small, imperceptible cracks develop
first and then steadily grow larger as firing
continues. This progressive stress damage
Anally results in tube rupture or in the
spalling of rifling lacds. When spalling occurs,
little effort from the projectile is needed to
remove the spalled section from the bore.
Such progressive stress am age limits the
length of the service life of the gun tube to a
prescribed number of flrings.
Another phenomenon that is unfavorable
to long tube life is erosion, the wearing away
of the bore surface. Erosion is primarily a
phyrictU activity caused by the abrasive
effects of the prooellant gases and of the
rotating band acting on the bore. The high
velocity propellant gases impinge on the bore
surface and sweep away some of the metal.
The intense heat of th~ gases contributes
indirectly to erosion by .^siting an extrr.iely
thin layer of the bore surface, thus making it
easier for gas to carry the metal away. The 3
high heat also causes some of the propellant
gas constituents to combine with the metal of
the bore surface. This newly formed com-
pound-normally a nitride and, therefore,
brittle-may crack and peel-off under the
action of the rotating bands and propellant
gases. At the origin of rifling, the tempera¬
tures are the highest and, thus, the erosion
rates are the highest. The rotating band has
two contributing influences on erosion; the
first induced by gas wash, the second by
ordinary sliding friction. However, these
influences are relatively insignificant to the
damage caused by propellant gases.
A significant amount of attention was
originally given to the problem of gas leakage
past the pre-engraved rotating band (Ref. 10)
and the erosive damage that could result. The
indicated problem was not received with
alarm, however, as the added clearance due to
the pre-engraving amounted to only one
percent of the bore area. As far as interior
ballistics is concerned, this clearance area was
equivalent to increasing the throat area by
approximately 1.5 percent, which is of the
order one would expect in variations of
manufacture. Further proof was substantiated
in the initial firings of the 57 mm Rifle, T15.
In the first 3,000 rounds for firing of a 57
mm Rifle, T15, no evidence of erosion was
found in the rifling.
As defined in par. 10-27, the tube is the
means for accelerating the projectile. The use
of a smooth bore definitely eliminates the
problems associated with a rifled bore such as
high friction, abrasion, and high resistance to
the projectile. However, the use of a smooth
bore requires a higher muzzlf velocity in
order to maintain the necessary aerodynamics
u>r the required projectile accuracy. But, in
considering the problems of gas leakage and
erosion which are still present in the smooth
bore rifle, the use of a smooth bore has found
very limited use in US recoilless rifle weapon
10-27
AMCP 706-238
systems, despite the fact that other nations
use them.
Once the decision to use a rifled-bore has
been made, it will be necessary to consider
the rifling shape and type of rifling twist to be
used in the tube. The shape of the rifling will
depend upon the type of rotating band to be
used. For projectiles with pie-engraved metal
rotating bands, it was found, during the 57
mm Rifle, T15 program, that shallow-groove
rifling will cause considerable shearing of the
rotating band and give a shorter accuracy life
of the tube. However, for rifles firing
fin-stabilized projectiles with plastic rotating
bands that are engraved during firing, shallow
groove rifling is used. In the U-BAT,
Super-PAT, and PAT rifle programs, for
example, a shallow groove rifling with a depth
of 0.006 in. is used. The use of shallow groove
rifling in these applications was made possible
by the use of fin-stabilized projectiles with
plastic rotating bands which are fired into a
rifle tube that has a slow rifling twist of
300-480 cal per turn. With this slow twist and
the use of plastic bands, the stresses on the
lands arc considerably reduced so that a
shallow groove rifling is possible.
In most recoilless rifle weapon systems,
uniform rifling twist is used since it meets the
desired requirements and is less expensive to
manufacture than a rifle tube with multitwist
rifling. However, multitwist rifling does
exhibit certain advantages over uniform twist
rifling. One advantage of multitwist rifling is
that, through the proper selection of the rate
of twist, the maximum stress on the lands and
rotating band of the projectile will be less.
This advantage is somewhat offset in recoilless
rifles as compared with conventional guns,
because they operate at lower maximum
pressures and muzzle velocities. Disadvantages
cf multitwist rifling arc: (1) higher and more
concentrated pressure on the driving edge of
the land and forward part of the band, (2)
increased friction and abrasion, and (3) higher
manufacturing costs. Weighing the apparent
advantages and disadvantages of both types of
rifling twist would seem to indicate that
uniform twist rifling is preferred for use in
recoilless rifles. For further discussion of
uniform versus multitwist rifling, die reader
is directed to the material found in Refs. 11
and 12.
During firing, the rifle tube is heated and
tends to droop as the temperature of the tube
approaches the tempera, are at which the
yield strength of the tube material begins to
decrease. The longer and thinner the tube, the
greater the droop of the tube. The extent of
the temperature-caused droop also will
depend on the elevation angle of the tube; the
higher the elevation angle, the lower the value
of the temperature at which droop begins.
The rifle tube also may bow to either side due
to differential heating during firing, if the
rifle tube is cooled on one side by action of
the wind, the bore axis will warp toward the
side from which the wind blows. The amount
of bowing will depend upon any waipage
initially in the tube, variations in wall
thickness, and the wind velocity.
As the projectile is accelerated through the
rifle tube, a torque is imparted to the rifling.
For uniform twist rifling, an approximate
expression for tlte rifling torque T is given by
Eq. 10-1 taken from Ref. 11.
tan a , in.-lb
( 10 - 1 )
where
p = polar radius of gyration of projectile,
in.
A b = bore area, in?
P g = propellant gas pressure, psi
R - radius of projecrile, in.
a = angle of rifling twist, deg
10-28
AJ*CT 706-238
SECTION VI
FIRING MECHANISM
10-30 OESIGN CHARACTERISTICS
The major components of the recoilless
rifle filing mechanism are the firing pin,
hammer, hammer spring, firing cable, trigger,
and safeties. The firing pin is a small diameter
rod with a hemispherical nose that generally is
stroked under the pressure of a stiff spring to
strike the primer of the chambered round of
ammunition. Some designs use the stored
energy in the spring to accelerate the firing
pin before striking the primer. Other designs
transfer all the stored energy in the spring to a
hammer. The hammer is accelerated and
strikes the firing pin that is positioned right in
front of the primer of the chambered round.
The firing pin generally is held in place by a
sear that is retracted or cammed away from
the firing pin to fire the round. The sear is
connected to the trigger of the rifle through
the use of a push-pull type control cable. The
stroking of this firing cable by the trigger
mechanism provides the necessary action for
releasing the sear from the firing pin.
The trigg :r designs for recoilless rifle use
have ranged from the fairly simple sear type
lever found in the 57 mm Rifle, Ml8, to the
fairly sophisticated trigger design found in the
120 mm Rifle, XM105, which provides for
the firing of the major and minor rifles from
the same trigger grip. Regardless of complex¬
ity, the design considerations to be made are
the ease in which the trigger is gripped and
the trigger pull squeeze force required to fire
the rifle. Human engineering requirements call
for a trigger pull of less than 40 lb, preferably
around 20 lb.
10-31 EXAMPLES
The firing mechanism operations of three
recoilless rifles are discussed in this paragraph.
In the 57 mm Rifle, Ml8, squeezing of the
trigger causes the firing cable to be pulled
forward (toward the muzzle). The firing cable
is connected to a sear lever (see Fig. 10-9) by
a safety spring. As the firing cable is pulled
forward, the safety spring causes the sear leyer
to rotate. The forked end of the sear lever
grasps the headed end of a sear which
prevents the striker assembly from striking
the cartridge primer. As the sear lever rotates,
it retracts the sear from a hole in the striker
assembly. When the sear is free from the
striker assembly, a compressed firing spring
drives the striker assembly (composed of a
hammer and firing pin) forward to strike the
orimer of the chambered round (Ref. 10).
The operation of the firing mechanism of
the 106 mm Rifle, M40, is similar to that just
described. Pushing the firing knob in the
elevating handwheel fires the rifle. The knob
is connected to the firing cable by a firing
transfer tang. Rotation of the tang actuates a
firing cable operating lever that strokes the
firing cable. At the breech end of the rifle, the
firing cable is attached to a trigger that rotates
as tht firing cable is stroked. Attached to the
trigger is a sear that prevents the firing pin
from stroking. When the trigger is rotated, it
causes the sear to tum and disengage from a
notch in the firing pin. Accelerated by a
spring, the firing pin strikes the primer of the
cartridge to fire the round (Ref. 13).
The firing mechanism of the 120 mm Rifle,
10-29
Figure 10-9. Firing Mechanism
XM105, incorporates a trigger mechanism
that serves both the major caliber rifle and
minor caliber spotting rifle. By twisting the
trigger grip shown in Fig. 10-10, the selection
of the rifle to be fired is made. Figs. 10-11
and 10-12 show the interior of the trigger
mechanism with the top plate removed and
indicate how the link is made to engage one
or the other bell-cranks which pull the firing
cables.
Operation of the trigger to fire the major
caliber rifle pulls the major caliber firing cable
(shown as the right cable in Figs. 10-11 and
10-12) forward. The major caliber firing cable
in turn, is attached to a firing trigger (shown
in Figs. 10-13 and 10-14) located at the
breech. The sear is engaged by the slot in the
firing trigger and is rotated by the firing
trigger to a fire position when the firing cable
is pulled. At the fire position, the sear releases
the hammer, which then is accelerated under
the action of a hammer spring to striice the
firing pin which, in turn, strikes the primer of
the chambered round (Ref. 14).
10-32 SAFETY DEVICES
The function of any type of safety used in
recoilless rifles is to prevent the gunner from
being able to fire the rifle until (1) the round
has been correctly chambered, (2) the breech
closed, and (3) the loader has given some type
of mechanical indication that the rifle is ready
AMCP 706-238
to be fired without endangering any equip¬
ment or personnel behind the rifle. In general,
two types of safety devices are incorporated
into the recoilless rifle design. The first safety
device is generally a part of the trigger
mechanism and consists of a lever or button
which v'hen depressed unlocks the trigger.
This safety feature prevents firing of the
weapon by any accidental contact with the
trigger.
The second safety generally is located at
the breech end of the rifle and is for the
loader’s personal safety. The safety device
usually is in one of two forms. One form of
the safety device consists of a simple
switching type device that is connected to the
firing cable. When the switch is in the “arm”
position, the firing cable is free to move as the
trigger is depressed by the gunner. In the
“safe” position, the firing cable is locked and
prevented from being stroked by any trigger
motion. The loader is then able to say, by his
choice of the safety position, when he feels
the gun is ready to be fired. A second form of
this type of safety is incorporated into the
breech mechanism. When the breech is open,
the firing mechanism is disconnected from the
firing cable. Not untii the breech has been
closed and locked completely will the firing
cable come into contact with the firing
mechanism This type of safety is found only
if the loader can close and lock the breech
from a position at the side of the rifle.
10-31
I
1
i
figure 10-11. Major Level of Actuation for Firing Mechanism, 120 mm RecoiUess Rifie, XM105
Figure 10-12. Minor Level of Actuation for Firing Mechanism, 120 mm Recoilless Rifle, XM105
St-o I
AMCP 706-238
1-HOUSING
2 -COVER
3- ROD
4- SPRING
5- TRIGGFR
Figure 10-14. Trigger Mechanism Components, 120 mm
'<K9.S(hiKw
AM CP 706-238
REFERENCES
1. Development of 120 mm Recoilless
Heavy Antitank Weapon System (HAW),
Final Report, Technical Memorandum
M64, Frankford Arsenal, Philadelphia,
Pa., 1 April 1959 through 30 June 1962.
2. Recoilless Weapons, Volume II, Nozzles,
Contract Nos. W-36-034-ORD-7652 and
W-36-034-ORD-7708, Franklin Institute,
Laboratories for Research and Develop¬
ment, Philadelphia, Pa., for Ordnance
Department, US Army, May 1948.
3. Symposium on Nozzle Design for Recoil¬
less Rifles, Held at Frankford Arsenal,
Philadelphia, Pa., 10 and 11 December
1951.
4. A. J. Grandy, Nozzle Spring Design for
90 mm Recoilless Rifle, T234, Memoran¬
dum Report M59-37-1, Frankford Arse¬
nal, Philadelphia, Pa., July 1959.
5. T114 Conference, Research and Develop¬
ment Program, Cadillac Motor Car Divi¬
sion, General Motors Corporation, 28
February 1961.
6. Recoilless Rifle Systems, Ammunition
and Related Items, Status Report No. 3,
Vol. IV, Report No. R-1366, Frankford
Arsenal, Philadelphia, Pa., 1 July through
30 September 1956.
7. C. W. Musser, et al., Strain Compensated
Barrels Report No. R-1008, Frankford
Arsenal, Philadelphia, Pa., May 1951.
8. Symposium on Recent Progress of Re¬
coilless Rifles and Ammunition, Held at
Midwest Research Institute, Sponsored
by Department of Army, 11-13 January
2954.
9. Robert Markgraf, Repeating Recoilless
Rifles, Frankford Arsenal, Philadelphia,
Pa., March 1960.
10. W. J. Kroeger and C. W. Musser, The
Design of Recoilless Infantry Weapons,
Report No. R-727, Frankford Arsenal,
Philadelphia, Pa., June 1946.
11. AMCP 706-252, Engineering Design
Handbook, Guns Series, Gun Tubes.
12. AMCP 706-108, Engineering Design
Handbook, Elements of Armament Engi¬
neering, Part Three, Weapon Systems and
Components.
13. TM 9-1000-205-12, Operation and Organ¬
izational Maintenance Cal. .50 Spotting
Rifle M8C; 106 mm Rifles M40A1 and
M40A1C; 106 mm Rifle Mounts T173
and M79; and Tripod T26, Headquarters,
Department of the Army, March 1959.
)4 120 mm Rifle System, XM105E1, Heavy
Antitank Weapon (Pi W) Notes on
Development Type Material, PDLWS-2,
Frankford Arsenal, Philadelphia, Pa.,
December 1962.
10-37
AMCP 706-238
BIBLIOGRAPHY
W. J. Hoff, “Boundary Value Problems of the
Thin-Walled Circular Cylinder,” Applied
Mechanics , December 1954.
W. J. Kroeger, The Piping of Recoilless Gun
Gases through Straight Channels and Bends,
Report R-860, Frankford Arsenal, Philadel¬
phia, Pa., July 1948.
J. J. Donnelly and G. Schecter, Firing Tests
for Discharging Nozzle Gases from Recoilless
Guns in Enclosed Installations, Report R-861,
Frankford Arsenal, Philadelphia, Pa., July
1948.
Merrill Eig and Harvey L. Peritt, Feasibility of
Glass-Filament Wound Plastics for Use in
Recoilless Rifles Such as 90 mm M67 , Techni¬
cal Report 3178, Picatinny Arsenal, Dov<?r,
New Jersey, April 1965.
AMCP706-23C
CHAPTER 11
AMMUNITION
11-0 LIST OF SYMBOLS
e r
=
energy required to raise surface of
the propellant to its ignition tem¬
A p
- total area of primer tube perfora¬
perature in time t t , ft-lb
tions, in?
F ,
ss
impetus of igniter material,
A ,
= surface area of propellant, in?
(ft-lbMlbr 1
a
= distance between two adjacent
S
=
acceleration due to gravity, ft-sec~ 3
perforations as shown in Fig. 11*1 S,
in.
h
=
length between perforations as
shown In Fig. 11-15, in.
- constant burning rate equation,
in.sec" 1
k
“
thermal conductivity of propellant,
(ft-lbHin? -sec- 0 K/in.)" 1
b
= cartridge case thickness, in.
L
=
length of propellant grain, in.
K
= equivalent unperforated cartridge
case thickness, in.
Wd)
=
length to diameter ratio of projec¬
tile, dimensionless
bi
s constant burning rate equation,
in.-sec" 1 -psi' n
M o
maximum end moment,
(in.-lbHn.* 1
C
= propellant charge weight, lb
N p
-
number of perforations in igniter
c,
= igniter charge weight, lb
tube, dimensionless
Ci
= propellant charge burnt, lb
=
weight of igniter gas produced up
to time t, lb
C P
(C P >.
= specific heat at constant pressure
of propellant (ft-lbHlb-°XF 1
= specific heat at constant pressure of
K
=
weight of igniter gas discharged
from igniter tube vents up to time
Mb
igniter, (ft-lbKlb^Kjr 1
(c v ),
- specific heat at constant volume of
n
=
combustion index, dimensionless
igniter, (ft-lbMlb- 0 KJ-*
n P
=
number of perforations in propel¬
D
= diameter of propellant grain, in.
lant grain, dimensionless
E a
- igniter gas energy made available to
P
=
instantaneous space-mean igniter
the propellant charge, ft-ib
tube pressure, psi
1-1
AMCP7M-US
P t * effective pressure acting over hex¬
agonal area, psi
P ( - pressure acting on inside of car-
tridge case, psi
P Q - pressure acting on outside of
cartridge case wall, psi
P p = peak pressure, psi
p c = chamber pressure, psi
Qo = end shear, lb-inT 1
r = perforated cartridge case radius, in.
r t = linear burning rate, in .-sec" 1
(r i ) l = linear burning rate igniter, in.-sec" 1
r p = perforation radius, in.
S. = instantaneous igniter surface area,
in?
T g = ambient temperature,°K
T e = effective nozzle thrust, lb
T, = ignition temperature of propellant,
°K
T 0 = isochoric flame temperature of
propellant or igniter, °K
K>
= time, sec
= ignition delay time, msec i
= internal volume of igniter tube, in. 3
= muzzle velocity, fps
= propellant web, in.
f3(l -
_ RiLzi&T'*
= L rj b z J ‘
1EL
-1
A P
y<
V
v*,v b
p
Pi
Oavg
differential pressure across webs of
perforated cartridge case, psi
specific heat ratio of igniter mate¬
rial, dimensionless
Poisson’s ratio, dimenisonless
constants from Ref. 1
density of solid propellant, lb-in." 3
density of igniter material, lb-in." 3
average stress, psi
a hoop = hoop stress, psi
a mx = maximum stress, psi
11-2
AM CP 706-238
SECTION I
GENERAL
11-1 INTRODUCTION
The major components of a round of
recoilless rifle ammunition are shown sche¬
matically in Fig. 11-1. The complete round
consists of the same basic elements as other
artillery ammunition; i.e., a projectile assem¬
bly, cartridge case, igniter assembly, and
propellant. The projectile consists of a
v/arhead for producing a destructive effect on
the target, a fuze for initiating the warhead at
the target, an aerodynamic envelope, and
either a rotating band to engage the gun
rifling if the projectile is spin-stabilized or fins
if the projectile *s fin-stabilized (or both if the
fin-stabilized projectile requires slow roll at
launch).
The design of a recoilless rifle projectile is
similar to designs for closed breech weapons,
with the exception of lower stresses due to
reduced pressures, accelerations, and engrav¬
ing loads. Also, the recoilless rifle cartridge
case differs from the conventional case in that
the wall is perforated to allow the efflux of
gases to a nozzle. There are some exceptions:
the 90 mm M67 System uses an unperforated
case that is supported by the chamber, and
the propellant gas path to the nozzle is
diivCtly rearward. Another exception is the
frangible plastic envelope used in one
approach to the 120 mm HAW project.
11-2 OVERALL DESIGN CONSIDERA¬
TIONS
In addition to designing recoilless rifle
ammunition for its terminal effects, consider¬
ation has to be given to the overall problems
associated with the assembly, loading, and
packaging of the round. Because of the
necessary close tolerances involved in recoil¬
less rifle weapon systems, it is important that
the assembled round fit correctly in the rifle.
In order that the rotating band of the
cartridge engage the barrel rifling effectively,
the correct alignment of the cartridge case
and the projectile must be maintained during
handling and final seating in the weapon.
During assembly of the projectile into the
cartridge case, it is possible-by crimping part
or all of the cartridge case mouth into a
groove machined into the projectile—to
maintain the necessary alignment between
cartridge case and projectile, and at the same
time, ensure that the cartridge case and
projectile will not separate under rough
handling conditions prior to and during
loading operations. The length of the crimp
may vary from a full crimp around the entire
periphery of the case mouth (ling crimp) to
small interrupted crimps made at equal
intervals around the circumference of the
cartridge case at a short distance from the
case mouth (stab and multiple dimple
crimps).
Before assembly of the projectile into the
cartridge case, a rubber-base cement may be
applied to the mouth of the cartridge case
prior to crimping the case to the projectile.
When the cartridge case is crimped to give the
assembly the necessary rigidity, the rubber-
base cement provides a reliable moisture seal
between the case and projectile.
After assembly, painting, and marking, the
comp te rouna is packaged to withstand
conditions usually found in the field. A round
is packed in an individual cylindrical
moisture-resistant asphalt (fiber) container
which in turn is packed in a sealed metal
container and then placed in a wooden box
for shipping.
Some of the more difficult problems
11-3
AMCP 706-238
related to loading have been encountered in
the assembly of fin-stabilized HEAT ammuni¬
tion. In most cases loading of the propellant is
accomplished through the mouth of the
cartridge case, but in most fin-stabilized
projectiles, it is necessary to load the
propellant through the base. Loading the
propellant through a single hole in the base
causes problems with packing the propellant
evenly around the fins of the projectile and
preventing any of the propellant grains from
entering into the primer retainer of the
projectile. When a base containing three holes
is used, the propellant can be loaded more
uniformly around the projectile fins. A more
uniform flow of the propel!wit grains during
loading of the fin-stabilized round is obtained
by vibrating the complete round as the
propellant flows into the cartridge case.
At times, it is desirable to be able to
position the propellant in specific areas of the
cartridge case because of the specific con¬
figuration of the boom of a fin-stabilized
projectile or due to the position of the holes
in the primer-igniter tube. For fin-stabilized
HEAT rounds, it is desirable to position the
propellant at the rear of the case; whereas in
spin-stabilized rounds, it may be desirable to
position the propellant along the perforated
section of the igniter tube. Plaster or cotton
and cardboard have been used to position the
propellant in the cartridge case.
Loading of the cartridge case should be
performed in the shortest time possible so as
to avoid the possibility of moisture absorp¬
tion by the propellant. On the other hand,
care must be taken to avoid damaging the
cartridge case liner that retains the propellant
in the cartridge case and acts as the
moisture-proof barrier for the propellant.
11-3 LIST OF EXISTING CARTRIDGES
WITH CHARACTERISTICS
Table 11-1 is a list of some existing
recoilless rifle cartridges with pertinent design
data.
11-5
TABLE 11-1
DATA FOR SOMh RECOILLESS RIFLE PROJECTILES
Complete Complete Cartridge
Muzzle Maximum Round Projectile Round Projectile Case
Velocity, Range, Twist, Weight, Weight, Length, Length, Length,
Stabilization fps yd cal/rev lb lb in. in. in.
Round Type Stabiliza tion fps
57 mm M13A1: 7,800 psi max. rated pres^jre (piezo)
M306A1
HE
Spin
1200 4930
30
5.46
2.78
17.54
6.47
12.0
M307A1
HEAT
Spin
1200 4860
30
5.43
2.75
18.78
8.07
12.0
M308A1
WP
Spin
1200 - 4530
30
5.43
2.75
17.54
6.43
12.0
75 mm M20: 10,800 psi max. rated pressure (piezo)
M309A1
HE
Spin
990 6960
22
22.37
14.40
28.92
15.10
16.0
M310A1
HEAT-T
Spin
1000 7300
22
21.06
13.19
28.9?
15.95
16.0
M311A1
WP
Spin
990 7020
22
23.20
15.10
28.92
14.54
16.0
M349
HEP-T
Spin
1400 7180
22
16.52
8.4
26.36
—
16.0
90 mm M67: 7,780 psi max. rated pressure (piezo)
M371A 1
HEAT
Fin
730 2200
160
9.25
6.43
28.00
23.00
16.35
105 mm M27A1: 10,000 psi max. rated press (piezo)
M323
HE
Spin
1120
20
49.15
32.40
40.73
15.74
M325
WP
Spin
1120
20
51.53
34.58
40.80
15.73
M326
HEP
Spin
1265
20
40.30
24.80
38.89
17.60
M341
HEAT
Folding Fin
1650
20
34.32
17.30
39.29
M34581
HEP-T
Spin
1690
20
38.00
17.54
38.00
106 mm M40A1: 11,000 psi max. rated pressure (piezo)
M344A1
HEAT
Folding Fin
1650
20
36.19
17.55
39.31
M346B1
HEP-T
Spin
1635
20
37.93
17.54
38.10
AMCP 706-238
AMCC 706-238
SECTION II
PROJECTILE
114 INTRODUCTION
The modem projectile used in recoilless
rifle weapon systems is cylindrical in shape
with a fairly long ogival nose. The projectile
will have either a boattailed (tapered) or
cylindrical (square) base and may be designed
with fins, depending upon the type of
stabilization required. The specific projectile
design depends very much on its function,
i.e., the type of target it is to defeat. As
discussed in Chapters 3 ana 5, the physical
characteristics of the projectile design con¬
tribute directly to the determination of both
the exterior and interior ballistics of the
weapon and to the dimensions of the overall
weapon system.
The relationship between the projectile
characteristics (mass, diameter, and shape)
and the overall weapon system is seen by
considering a weapon system in which it
desired to extend the projectile range. Since
requirements specify the defeat of a certain
target, the projectile characteristics are not
changed. Thus, the only means of achieving
the increased range are by increasing the
chamber pressure, increasing the barrel length,
->r using a combination of the two. it is
evident that any of these changes will result in
a larger, heavier weapon, if the designer is
limited to the use of specific weapon
materials.
If it were possible to change the projectile
characteristics while achieving the same
terminal effects, it is evident that compro¬
mises would have to be made in other aspects
of the weapon performance. Increasing the
diameter of the projectile while maintaining a
constant weight may result in a lower
chamber pressure to achieve the same muzzle
velocity, but will result in a shorter trajectory
because of the effect of increase i projectile
diameter on the exterior ballistics. From this
short discussion, it can be seen that in the
projectile design, it will not be possible to
consider only the desired terminal effects;
how the projectile can be dimensioned to
achieve a minimum weight gun while still
effectively defeating the specific target must
be considered also.
11-5 PROJECTILE TYPES
Projectiles commonly are classified accord-
in e to the type of warhead employed and the
tactical use for which the projectile is
intended. The various types of warheads have
been described in detail in Chapter 3. Figs.
11-2 through 11-7 show simplified cross
sections of various types of projectiles that
are described brietly as follows:
1. High explosive Antitank (HEAT), Figs.
11-2 and 11-J. Designed for the defeat of
armor, this projectile uses a hypervelocity,
shaped-charge jet formed by interaction of
explosive charge and liner to penetrate tank
armor. Since spin degrades penetration by
“defocusing” the jet, this projectile common¬
ly is fin-stabilized.
2. High Explosive (HE), Fig. 11-4. De¬
signed for defeat of personnel, materiel, and
field fortification through blast and fragmen¬
tation, the HE projectile consists of an
explosive charge contained in a thin-wali, steel
envelope. Depending upon the terminal
ballistic requirements, various combinations
of fragmentation and demolition effects can
be obtained by proper selection of wall
thicknesses and explosive charge size. Projec¬
tiles designed for fragmentation effects have
thicker walls and smaller bursting charges
than projectiles designed for demolition use.
The standard envelope consists of a 2- to
3-caliber tangent ogive and a 2- to 3-caliber
11-7
Figure 11-2. Folding Fin HEAT Projectile
Figun 11-6. WP Project!/*
cylindrical body. Depending upon the ex¬
terior ballistic requirements, the projectile
may have a O.S caliber boattail.
3. High Explosive Plastic (HEP), Fig. 11-5.
The HEP projectile is designed to defeat
armor through the spalling of the rear face of
the armor by action of shock waves produced
by detonation of the projectile explosive
charge on the amor front face. The fuzing
consists of an inertia type, base-detonating
fuze timed to detonate the explosive after it
has spread out (squashed) on the front face of
the armor. The projectile design consists of a
1-caliber tangent ogive and a 2.5- to
3.5-caliber cylindrical boay with a threaded
base plug containing the base fuze.
4. Incendiary or White Phosphorus (WP),
Fig. 11-6. Designed to provide smoke
screening and incendiary effects, the WP-type
projectile usually consists of HE projectile
metal parts redesigned to accept a press-fit
central burster tube with an enlarged mouth
diameter threaded internally to accept the
fuze.
5. Antipersonnel (APERS). The antiper¬
sonnel projectiles include all types of
projectiles designed to release lethal subpro¬
jectiles upon actuation or detonation. Canis¬
ter and beehive projectiles are examples of
antipersonnel type projectiles.
The canister type projectile (Fig. 11-7) is
designed for the defeat of personnel in man
attack by functioning immediately in front of
the weapon to release a large number of lethal
subprojectiles. The projectile consists of a
cylindrical container filled with small slugs,
balls, or flechettes with a thin closure crimped
on the forward end. The walls are extremely
thin so that they will fail due to centrifugal
force upon emergence from the weapon.
Longitudinal grooves are machined on the
outside of the wall to assist failure of the
walls. Dispersion of the payload is achieved
by centrifugal force and, therefore, is a
function of muzzle velocity, subprojectile
masses and spin. The lethal masses appear in
the form of a cone with its apex at the muzzle
of the weapon.
The beehive projectile, designed for the
defeat of personnel in mass attack from range
zero to maximum range of the weapon
through the release of flechettes at the desired
range, is basically a canister projectile fuzed
to function at specific ranges beyond range
zero. The fuze is a mechanical time (MT) fuze
that can be set for either muzzle action or
time increments of 100 m,
11-9
AMCP 706-238
11-8 DESIGN CONSIDERATIONS
11-6.1 ENVELOPE
in the design of the projectile envelope, the
designer must consider the different effects
that tiie firing, trajectory, and impact stages
of the mission have on the projectile. These
considerations wiil involve ihe structural
strength, aerodynamics, and the terminal
ballistic effects of the projectile envelope.
1. Aoility To Withstand Stress During
Firing. Regardless of the function of the
envelope as determined by ‘he type of
projectile, the envelope must show the ability
to withstand the stresses sustained during
tiring (metal parts security). These stresses are
a result of the propellant gas pressure acting
on the base and the wall behind the projectile
rotating band, and the acceleration and
rotational forces acting on the entire en¬
velope. Since the maximum rated pressures of
recoilless rifles are considerably lower than
for closed breech systems, the designer has
greater latitude in the choice of steel and
aluminum alloys, and also in the use of other
materials such as plastics, reinforced plastics,
and Fiberglas. A stress analysis on the
envelope configuration will establish the
specific strength requirements of the materials
to be used and confirms the adequacy of the
sections to withstand the firing stresses.
2. Aerodynamic Stability. The second area
of projectile design is making the projectile
aerodynamically stable in order to provide a
controlled and predictable flight to the target.
The method or type of stabilization-either
spin or fin, or a combination of both-is based
on the primary warhead type and configura¬
tion, and the effect of spin on the warhead.
For spin-stabilization, twists of rifling of 1
turn in 18- to 30-calibers of travel are used.
Once the type of stabilization has been
established, satisfactory aerodynamic stability
can be achieved by proper mass distribution
within the projectile envelope. In the absence
of aerodynamic stability, the net effect of the
aerodynamic, inertia, gravitational, and gyro¬
scopic (for spin-stabilized projectiles) forces
acting on the projectile will be to increase the
oscillatory motion of the projectile until it
begins to tumble.
3. Terminal Ballistic Effect. The envelope,
while being structurally sound and dynamical¬
ly stable, also may be a contributing factor to
the terminal ballistic effect and is a third
consideration in projectile design. In the case
of canister and beehive-type projectiles, the
envelope serves only to carry the warhead
from the launching weapon to the target,
whereas the warhead of a high explosive type
projectile contributes directly to its fragmen¬
tation effect. In the case of WP projectiles,
the envelope makes a secondary contribution
in that the dispersion and burning effects of
the WP are controlled by the manner in which
the envelope fragments upon detonation.
The optimization of all three areas of
projectile design discussed in this section
usually is not obtainable, and leaves the
designer faced with the choice of making
trade-offs to achieve the specified military
characteristics.
11-6.2 REQUIRED INFORMATION
In order to prepare initial projectile
design, the designer must be supplied with the
following information as determined by
analysis of the specified performance require¬
ments:
1. Warhead Type. The designer is informed
of the category of primary target to be
defeated in order to determine the type,
configuration, and quantity of explosive or
chemical charge.
2. Caliber. The caliber of a recoilless rifle is
the diameter of its bore. The term caliber also
may be used as a unit measurement in
expressing the rifle length. Specifying the
caliber of the rifle determines the projectile
diameter
AMCP 706 238
3. Projectile Weight. As determined by an
overall system analysis to achieve the lightest
weight system compatible with range and
terminal ballistic requirements the projectile
designer will h supplied with a maximum
projectile weight. This information, along
with the maximum chamber pressure, will
enable the designer to properly size the
projectile wall thicknesses and choose mate¬
rials with the necessary strength requirements.
4. Maximum Chamber Pressure and Rifling
Twist. Specification of the maximum cham¬
ber pressure and rifling twist enables the
projectile designer to perform the necessary
stress analysis, to establish strength require¬
ments, and to assure that the projectile will
withstand the stresses sustained during firing.
5. Muzzle Velocity. In order wo evaluate
acrodynamically the projectile design and
show that the projectile will meet minimum
trajectory requirements, the designer must be
provided with the muzzle velocity. Given the
muzzle velocity, the designer can determine
the necessary aerodynamic coefficients needed
to perform trajectory and stability analyses.
11-6.3 METHOD OF STABILIZATION
The method of stabiliza ion is based on the
primary warhead type and the effect of spin
on this type of warhead. As stated in par..
11-6.1, spin-stabilization is preferred because
it provides the best accuracy and is the most
economical round to manufacture. However,
if defeat of armor is the primary purpose of
the system, a HHAT warhead is then required,
necessitating the use of fin-stabilization.
Fin-stabilization is preferred for the HEAT
warhead since the jet is affected adversely by
spin. Even with flow-turned liners producing
spir compensation up to approximately 30
rev per sec or with fluted designs (up to
approximately 100 rev per sec), a certain
amount of penetrution is lost compared to a
nonspinning warhead.
In recoilless systems with the exception of
the original 55 mm and 75 mm systems, the
HEAT projectiles are fin-stabilized to obtain
maximum armor penetration for minimum
caliber and weight. However, to improve
accuracy even the fin-stabilized rounds are
given a slow spin comparable to 1 turn in 160
to 250 calibers of travel. In imparting spin to
a fin-stabilized projectile, the spin must be
high enough to avoid resonance and low
enough to avoid inagnus effects. Generally, a
spin rate between 10 to 20 revs per sec will
avoid either of these conditions.
If the purpose of the system is both
r.ntiarmor and antipersonnel for which fm-
ar.d spin-stabilization, respectively, are pre-
ferred-thc latter particularly for canister- and
bechivc-type projectiles--the designer must
make a choice in favor of one or the other.
Generally, spin-stabilization is chosen and the
effect of spin on the fin-stabilized HEAT
round minimized by designing the HEAT
round without an obturating band. Thus, spin
transmission to the finned round is only
through friction oi the obturating band on
the projectile body against the rifling which is
usually equal to or less than 30 revs per sec for
which a flow-turned liner can provide
adequate sp: ^ compensation.
11-7 METAL PARTS SECURITY—STRUC¬
TURAL INTEGRITY WITHIN THE
BALLISTIC ENVIRONMENT
11-7.1 GENERAL
Upon completion of the design
layout that establishes the ..v • , or.figura¬
tion that satisfies the w «b a '/eight and
material requirements, a • * analysis is
conducted. This analysis establishes the
strength requirements of the materials and
confirms the adequacy of the section
thicknesses chosen 10 withstand the Firing
stresses. Based on the results of the analysis,
the envelope (section thicknesses) may have
11-11
AMCP 70*238
to be redesigned to bring the stress levels into
practical limits for the commonly used
materials, he., steel and aluminum. The
desired limits for steel are yield strength of
60,000 to 90,000 pr for which low to
medium carbon steels can be used; the limits
for aluminum are yield strengths of 40,000 to
70,000 psi for which, in order of descending
preference, the following alloys can be used,
6061-T6, 2024-T3, 2034-T6, 7079-T6, and
7075-T6.
Since the maximum rated pressures of
recoilless rifles are considerably lower than
for closed breech systems, the designer has
greater latitude in the choice of steel and
aluminum alloys, anu also in the use of other
materials such as magnesium, plastics, rein¬
forced plastics, and Fiberglas. In choosing
materials, the designer should always keep
cost in mind and use the most economical
materials that will satisfy the structural,
aerodynamic, and warhead requirements. For
fixed-fin rounds, the fins usually are extruded
aluminum. Additional material considerations
are included under aerodynamic a. d warhead
design.
11-7,2 STRESS ANALYSIS
< iic accepted rw ;thod of stress analysis is
based on thin-wan shell theory specifically
adapted to the conditions of projectile design.
The analysis is conducted at specific critical
points based on 115% of the maximum rated
pressure and minimum metal conditions to
ensure an adequate factor of safety under all
temperature and pressure extremes.
The critical points are:
1. The capability of the center of the base
and the intersection of the wall to withstand
the propellant gas pressure and the setback
pressure of the filler
2. The capability of the wall immediately
behind the roti ng band to withstand
propellant gas pressure, and the acceleration
and rotation forces of the body and the finer
3. The capability of the wall immediately
forward of the rotating band, the intersection
of the bourrelet and the ogive, and the thin
section of the ogive to withstand the forces of
acceleration and rotation of the body and the
filler
4. The capability of all threaded joints to
withstand, in addition to the forces of
acceleration and rotation, those forces due to
relative distortion of the mating parts.
Although no analysis is conducted for the
forces applied during the engraving of the
rotating band, it r recognized that these
forces exist and that temporary and/or
permanent deformation of the body can
occur due to band engraving. Particular
attention must be paid to this during the
initial metal parts security tests and if
excessive deformation is evident (greater than
or equal to 0.005 in.), the design must be
modified to eliminate it
To avoid this body deformation problem
and the associated problem in the weapon
design, pre-engraved rotating bands generally
are used for recoilless projectiles. However,
when a pre-engraved band is used, it presents
two other problems; i.c., lack of obturation
(propellant gas leakage past the band) and the
need for indexing the band into the rifling
during loading. The first problem is of direct
concern in the stress analysis in that the
critical sections forward of the band are
subjected to the propellant gas, a factor that
must be included in the stress analysis. This
problem can be alleviated by the use of a
plastic or rubber obturator. However, these
materials are quite sensitive to the effects of
temperature extremes, and therefore, are not
100 percent reliable. Consequently, even
when an obturator is used, it is good practice
to assume leakage will occur, thus applying
propellant gas pressure to the body sections
forward of the pre-engraved rotating band
and/or obturator.
The formulas, as corrected empirically,
presently used in projectile design are given in
AMCP 700-238
Pef. 11. The formulas will give the longi¬
tudinal, radial, and tangential stresses of each
section and are combined to give the resultant
stress in accordance with the Hencky-Von
Mises theory of failure.
11-8 AERODYNAMIC DESIGN
Upon establishment of the design param¬
eters including method of stabilization, the
initial envelope can be designed by methods
of stress and aerodynamic analyses. Detailed
definition of the aerodynamic design param¬
eters, criteria, requirements for stability, and
method of analysis are given in Chapter 4.
Generally, for either spin- or fin-stabiliza¬
tion, satisfactory stability can be achieved by
the proper distribution of the mass. For a
spin-stabilized projectile the designer should
attempt to maximize the axial (polar)
moment of inertia and minimize the trans¬
verse moment of inertia. This usually can be
achieved with a one-piece steel envelope with
properly contoured walls. However, if the
ratio of length to diameter (i/d) exceeds 4 to
4.S calibers, this may not be possible and
multipiece construction employing light¬
weight materials for the ogive and bast may
be required. Beyond 6 to 6.S calibers even
multipiece construction will not provide
sufficient gyroscopic stability, and fin-stabili¬
zation will have to be used if the warhead
requirements dictate a longer projectile. Ref.
14, even though quite old, can be used to
good advantage.
11-9 OTHER DESIGN CONSIDERATIONS
Within the general design considerations
discussed in par. 11-8, there are several
specific considerations which deserve mention
in this paragraph.
1. Joints When HE Is Used. Multipiece
construction with joints exposed to the high
explosive never should be used in the
projectile design. Inspection costs alone
would dictate against this practice, but there
is always the very real danger of an
inadequately inspected projectile being loaded
and fired. An in-bore detonation of the HE i3
too high a price to pay for the design
alternative. Projectiles should not be designed
with joints in a ' area to be occupied by HE.
Press-fit joints are used on US Army
projectiles to seal WP within the projectile
body; the British use threads coated with a
sealant for the same purpose.
2. Bourrelet Design To Minimize Balloting.
Another design consideration is the projectile-
rifle relationship during the travel of the
projectile through the tube. In-bore yaw
(balloting) should be held to a minimum
because it results in excessive yaw at muzzle
exit (aerodynamic jump). Two widely spaced
bounrelets (bore riding surfaces) fore and aft
of the rotating band are desirable with
minimum clearance between the land and
bourrelet diameters. Present practice defines
minimum bourrelet clearance as 0 002 in. plus
0.001 times the caliber in inches, with a
minimum of 0.004 in. for 37 mm and smaller
projectiles. A tolerance of plus zero minus
0.00S in. is specified for all projectiles. Thus,
projectiles may be designed for a maximum
clearance of 0.007 in. plus 0.001 times the
caliber in inches; or for 37 mm and smaller,
0.009 in.
3. Avoidance of Abrupt Surface Irregulari¬
ties. Abrupt surface irregularities also should
be avoided. At any joint, the forward section
always should be flush with or slightly larger
than the mating part. This should be
maintained under all conditions of tolerance
and ovality to prevent sharp, flat, drag-pro¬
ducing projections into the airstream (com¬
monly referred to as the “reverse umbrella
effect”).
4. Prevention of Flow Separation. Ideally,
there should be no separation of the airstream
over the projectile envelope during flight. If
erratic flight does occur at some point in the
trajectory, it is most likely due to airstream
separation, or spin-yaw resonance, and/or
m wwas
AM CP 706-238
magnus instability. This is particularly true
for fin-stabilized projectiles.
5. Boom Design for Fin-stabilization. In
the design of fin-stabilized projectiles, the
length of boom connecting the fin to the
projectile body is limited by the gun chamber
configuration. In many instances, use of the
boom to contain the igniter tube requires a
longer boom than necessary for projectile
stability. In addition to placing the fin in a
more rearward position than required for
stability, and emphasizing the attendant yaw
and pitch perturbation effects during launch,
the tube also is subjected to bending stresses
(flexure) due to unequal in-bore pressure
distribution and vibrations at launch and
during flight.
6. Typical Fin Design. For full caliber
fixed-fins, six-bladed extruded T-section fins
have been used successfully. The T-section
configuration (also referred to as an end
plate) is used to maximize the effectiveness of
the fin by deterring the movement of air over
the edge of the fin from the high pressure to
the low pressure side. The end plates also add
to the torsional rigidity of the fins, providing
a larger bore riding surface during launch and
preventing fins from entering into rifling.
In the use of folding fins, the designer
should design the mechanism for opening the
fins to ensure simultaneous opening of all of
the blades. In the 106 mm Projectile M344,
the fins are opened by the entrapped gas
pressure acting on a piston geared to all fin
blades.
1 ■ Spiked Nose. If the range requirements
arc sufficiently short and the muzzle velocity
high enough that a spiked nose configuration
can be used, particular attention must be paid
to spike diameter and length to avoid a dual
flow condition. Dual flow is defined as the
reattachment of the shock wave off the tip of
the spike to the cylindrical portion of the
spike. Dual flow significantly increases drag
and is usually an inconsistent phenomenon
from round to round. Experience indicates
dual flow generally can be avoided by the
placement of a trip ring approximately 1.3 to
1.4 times the spike diameter and located
approximately 1/3 caliber from the tip of the
spike. A trip ring is a sharp expansion in the
diameter of the spike, i.e., a ring or a flange.
8. Automatic Indexing Device. Experience
with recoilless rifles has indicated that in
loading a round into the weapon, difficulty
often was encountered in registering the
engraving of the rotating band with the lands
and grooves of the rifling. Therefore, it was
necessary to enter the round up to the
rotating band and then slowly rotate the
round until the proper location was found
before the round was driven home. In order
to chamber the round in one uninterrupted
motion, a means for automatically indexing
the projectile is needed. Automatic indexing
of the projectile is accomplished by the use of
two buttons located 180 deg apart on the
projectile bourrelet with their center points
on the centerline of the rotating band tooth
helix. The buttons are machined of brass or
similar material to the proper diameter in
order to engage the groove in the rifling. The
buttons are then crimped into holes in the
bourrelet and provided with a spring or
cushion that permits the buttons to compress
into the projectile if they first strike a land
before expanding into the rifling groove.
After these buttons engage the rifling grooves,
they force the round to rotate with the rifling
and thereby properly register the pre-engraved
rotating band into the rifling. The indexing
button also may be a simple flat (leaD spring
fastened to the body in line with a given land
of the rotating band co that the spring will
expand into the rifling groove when loaded
into the rifle.
9. Avoidance of Mass and Configurational
Asymmetries. As stated previously, a slow
spin is desirable for fin-stabilized projectiles
to reduce the effects of asymmetry. Although
initial spin can be imparted by the rifling, it is
often necessary to maintain the spin through¬
out the flight of the round. This can be done
by either beveling the leading and/or trailing
11-14
AMO* 706-238
edges of fin blades or by canting the fin
blades. The mqjor difficulty in employing
either of these methods in mass production is
controlling the degree of bevel or cant within
the closely toleranced limits required for spin
control.
11-10 WARHEAD DESIGN
In general, in order to achieve a desired
terminal ballistic effect, the designer is faced
with the problem of packaging the largest size
warhead in an envelope that is structurally
adequate and aerodynamically stable. Specifi¬
cation of the projectile type usually dictates
the warhead size, shape, and type of explosive
charge or chemical filler to be used, but also
will present design problems inherent with a
projectile type. Detailed design parameters
and criteria for terminal ballistic effects for
the various types of projectiles are given in
Chapter 3.
11-11 ROTATING BAND
The purpose of the rotating band is
threefold: to center the projectile, impart
spin, and provide obturation. However, for
recoilless projectiles, a pre-engraved band
normally is used for spin-stabilized projectiles,
i.e., the band is machined to match the rifling
in the tube. Consequently, the band provides
only two of the functions and if maximum
efficiency is to be obtained, a separate
obturating band is required.
The pre-engraved rotating band may be
machined directly from a raised projection of
steel integral with the body or machined from
another material which is swaged, threaded,
or welded to the body prior to machining.
For example, on the thin-wall HEP projec¬
tiles, the band is machined from a copper
overlay welded onto the body. Clearance
between the band and the rifling is held to a
minimum to minimize propellant gas leakage.
To facilitate engagement of the pre-engraved
band into the rifling when loading the
cartridge into the weapon, an indexing button
on the forward portion of the projectile body
may be used. If the complete round length is
short, as in the 57 mm and 75 mm systems,
this is not required. However, in 106 mm or
larger calibers, it generally is required.
A pre-engraved band is not used for
fin-stabilized projectiles fired from rifles
having a slow twist. Instead, a thin plastic
band, Geon or nylon, shrunk into a shallow
smooth seat is used. The forces applied to the
projectile wall during engraving of the plastic
band are minimal and usually can be ignored.
In addition to transmitting spin, the plastic
band serves as an obturator. An example of
this is the M371 HEAT Cartridge for the 90
mm Recoilless Rifle.
By considering the advances being made in
plastics, it may be possible for future systems
to use plastic bands for spin-stabilized
projectiles, thus eliminating the cost of
pre-engraving and associated problems of gas
leakage and indexing.
11-12 OBTURATORS
Since the pre-engraved bands do not
provide obturation, it is often desirable to use
an auxiliary obturating band. This band
usually consists of a rubber or plastic ring
located behind the rotating band. The rear
edge of the obturator may be flared outward
to assist the propellant gas pressure in forcing
the materia) into the rifling. However, this
generally is not required to achieve an
effective seal.
Many baud geometries are satisfactory, and
a groove or seat may or may not be required
to position the obturator while in the tube.
This is left to the discretion of the designer
based on test results. Of particular concern,
however, is the choice of materials that will
perform satisfactorily over the temperature
extremes of —65° to >40°F. One hundred
percent reliability under all temperature and
pressure conditions rarely can be achieved.
AMCP 700-238
11-13 STRAIN COMPENSATION
Because of the extremely thin walls of
recoilless rifles, deformation of the rifle due
to gas pressure and the attendant increase in
bore diameter are of considerable concern to
both the projectile and rifle designers. To
compensate for the expansion of the bore, the
diameter of the rear bounelet is increased to
maintain the desired fit between the i rojectile
and the bore during travel through the tube.
If an obturator is used, it also is designed for
the oversize bore condition. Consequently,
even when a plastic rotating band is used in
lieu of a pre-engraved band, it may still be
necessary to use a separate obturating bend.
The amount of expansion usually is deter¬
mined by the rifle designer and supplied to
the projectile designer. The 90 mm M67 Rifle
and M371 Projectile combination is an
example of a strain compensated system.
11-14 SHOT START
This is the force or chamber pressure
required to start the projectile moving. A
minimum shot-start pressure is required to
achieve proper ignition of the propellant
particularly when pre-engraved rotating bands
are used. Generally, sufficient shot start can
be obtained by crimping the cartridge case
tightly to the projectile. Forces of 8,000 to
10,000 lb can be obtained with a good crimp.
When sufficient shot start cannot be obtained
through crimping, a special tensile connection
usually is made between the primer and the
base of the projectile via a threaded joint.
This is particularly true in fixed fin-stabilized
projectiles in which the projectile boom and
fin assembly are used to contain the igniter.
11 16 SPIGOTS
A spigot weapon system-whethcr it be
mortar, closed breech, or recoilless-consisis
of an over-caliber warhead mounted on a full
caliber type (spigot) that is inserted into the
rifle barrel (muzzle loaded). The spigot may
extend partially down the barrel or all the
way to the chamber. The joint between the
projectile base and the spigot may be either
fixed or loose to permit separation (discard)
of the spigot in flight. The basic purpose of
the spigot is to permit the launching of large
over-caliber warheads from lightweight, small,
weapon systems. An example of a spigot
system is the DAVY CROCKETT rc coilless
weapon system (see Figs. 1-13 and 1-14 in
Chapter 1).
AMCC 706-238
SECTION III
CARTRIDGE CASE
11-16 INTRODUCTION
Recoilless rifle cartridge cases differ from
ordinary closed breech cartridge cases due to
the requirement that there must be provision
for rearward flow of the vented propellant
gases. The two most common chamber
configurations are the breechless-axial nozzle
rifle and the breech loading-multiple nozzle
rifle; both of which vent from the rear of the
rifle. Another poss-ble configuration is the
breech loading-forward orifice rifle that may
vent rearward from the sides of the chamber
or even forward of the chamber itself. These
three configurations are shown in Figs. 118,
11-9, and 11-10.
In these figures, it is shown that for the
axial nozzle weapon, the rifle is usually
breechless and the function of the case is that
of a container and loading tool for the
explosive. It is, therefore, desirable for the
case to be disposed of during the firing cycle
so that cartridge case ejection is not required
as a separate function. Two systems—rifles
incorporating combustible or frangible car¬
tridge cases-attempt to accomplish this
function. In the combustible cartridge case,
the case bums and contributes to the
propellant charge, whereas the frangible
cartridge case disintegrates and “blows” out
through the nozzle.
11-17 THE PERFORATED CARTRIDGE
CASE
11-17.1 GENERAL
The main advantage of using a perforated
cartridge case is the rapid radial expansion of
the propellant gases into a relatively large
chamber volume. Since the total perforation
area greatly exceeds the bore or thioat area,
the case offers little resistance to the gas flow,
and enables the case to be made considerably
smaller than the gun chamber. The lighter
perforated cartridge case is considerably easier
to handle than a comparable closed-breech
cartridge case.
While the use of a perforated cartridge case
results in considerable weight savings, there
are several problems inherent in its use. One
problem in the design of the perforated
cartridge case has been the selection of case
material of suitable hardness which will
ensure that the case deformation resulting
from firing will be small enough to allow free
extraction of the case. Another problem is the
selection of a material to cover the perfora¬
tions in the cartridge case for use with
fin-stabilized projectiles and the method by
which this liner is applied to the inside of the
case. The liner must be able to withstand all
the environmental and handling conditions to
which the case is exposed while (1) providing
an adequate moisture seal for the propellant,
(2) exhibiting sufficient strength to prevent
the propellant grains from punching through
the liner, and (3) leaving no undesirable
residue in the weapon after firing.
Cartridge case perforations have been
scaled with various materials. Originally,
paper was used, then polyethylene and
cellulose nitrate. Now, the preferred r. sthod
of sealing the cartridge case perforations in
fin-stabilized rounds is through the use of
polyethylene terephthalate (PETP)* film. In
spin-stabilized ammunition, the propellant
generally is contained in a polyethylene/rayon
laminated bag.
In applying a PETP liner, the PETP film is
precut and preshrunk, and then placed inside
of the cartridge case which has been covered
with a suitable adhesive. A rubber mandrel
then is inserted inside the liner and expanded
so that the liner is pressed tightly against the
•MyUi (Type A), E. 1. DuPont, DeNemours i Co.
11-17
AMCP 706-238
Rearward
Rearwari
Gas Venting
SC
Chamber propellant Projectile
Nozzle J j j
Barrel
f f- Gas Expands
i ^ in all Directions
J~'
Figure 11-10. Axial Nozzle—Combustible Case
11-18
AMCP 706-238
inside of the cartridge case. This procedure is
described in more detail in par. 11-17.8. The
resulting PETP lined case provides the
necessary moisture seal while being of
sufficient strength to prevent propellant
grains from punching through the PETP film.
Manufacture of the perforated cartridge
case has been accomplished by two different
techniques. The preferred method of manu¬
facture is to roll perforated sheet steel into a
cylinder and attach it to a machined head by
a copper braze. This method of manufactur¬
ing has superseded a previous, more costly
method that used a drawn steel case
manufactured in a manner similar to artillery
cartridge cases. Prior to tapering and necking,
the required number of perforations is
punched out one row at a time.
11-17.2 EFFECT ON INTERIOR BALLIS¬
TICS
In the design of a perforated cartridge case,
its effect on ballistic performance is of prime
importance. Ballistic performance includes
factors such as:
1. Rearward recoil
2. Peak chamber pressure
3. Propellant loss.
These characteristics are dependent upon the
cartridge case perforation parameters, as well
as those associated with the charge and
chamber-nozzle configuration.
Determination of the dependence of the
internal ballistic characteristics of the weapon
upon the cartridge case perforation param¬
eters is a complex problem, which in prior
practice, mainly was arrived at experimental¬
ly. A specific set of numerical results is shown
in par. 11-17.3 for the 57 mm M30A1B1 Case
tested with file Ml8 Recoilless Rifle to
illustrate the relative performance expected in
varying the perforation parameters.
The data for Fig. 11-11 were obtained for a
variation in percent perforation area and a
corresponding variation of charge necessary to
obtain a muzzle velocity of 1150 fps. It will
be noted that as the percentage perforation
increases, the charge required to maintain the
1150 fps muzzle velocity increases. This is
due to the increased propeliant loss and
decrease in peak chamber pressure.
It also can be seen that for a 100 percent
perforation area (zero strength cartridge case)
the rearward recoil is minimized with a
corresponding increase in the charge required.
At the other extreme, approaching a zero
percent perforation area (this approaches the
performance of a closed-breech weapon), the
charge requirement drops off sharply (in¬
creased efficiency) and the recoil increases
greatly. It is to be understood that a balance
between recoil and ballistic efficiency has to
be chosen, with the understanding, for
purpose of discussion, that an increase in
charge simply indicates a decrease in effi¬
ciency.
11-17.3 EFFECT OF PERFORATION
HOLE DIAMETER
Fig. i 1-12 shows another set of data. In
this case, both the charge and perforation
hole diameter were varied while maintaining
the 1150 fps muzzle velocity. In addition, the
total perforation area was held constant at 50
percent. Thus, the hole spacing was increased
as the hole diameter was increased, si that
percentage of area effects will not be
interpreted as hole-diameter effects.
The general trend of the curves in Fig.
11-12 is similar to that of the Fig. 11-11
curves with the same principles applying,
except that the limitations on the extremes of
hole size are more severe than those which
apply to percentage perforation area. If the
hole diameter approaches or exceeds the
propellant grain length, the loss of unbumt
propellant will be excessive. If, on the other
hand, the hole diameter is reduced very
Perforation Area/Throat Area
iMwa.ariiiMmf-aiK.
AMCP70G-238
greatly, the wet thickness between holes
must, correspondingly, be made very small in
order to maintain a constant SO percent
perforation area.
The foregoing information is presented for
the purpose of indicating the trends of the
experimental results which may be expected.
Likewise, the results that follow indicate the
trend of effects which were found by varying
the arrangement of the perforations (which
were assumed to be arranged in a uniform
pattern).
The effects of perforation disposition
were determined by comparing three types of
distribution:
1. Type I. holes evenly distributed over the
normally perforated section
2. Type II. holes concentrated in the
breech half of the normally perforated section
3. Type III. holes concentrated in the
muzzle half of the normally perforated
section.
All three cases were perforated with 200 holes
of 9/32 in. diameter (50 percent of the
perforation area of the M30A1B1 Cartridge
Case). The hole pattern for Types II and III is
identical to that of the M30A1B1 Cartridge
Case, except that the muzzle and the breech
sections of these respective cases are not
perforated.
Table 11-2(A) contains the experimental
data for the three cartridge case types. The
unperforated section of the Type III cases
ruptured when fired; the perforated portion,
on the other hand, remained intact, and
showed no apparent deformation.
Judging from the amount of recoil
unbalance and the pressures obtained for the
various cases, there are indications that the
flow of recoil-compensating gases is somewhat
concentrated to the rear of the chamber,
rather than evenly distributed. For the Type
III case, where the gases were forced to flow
forward in the cartridge case, the perforation
pattern resulted in high pressures, failure of
the cartridge case, and high recoil unbalance.
The test results show that the propellant
losses are roughly inversely proportional to
the average distance the propellant grains
must travel to be ejected, and that the major
portion of the flow is concentrated at the
nozzle end of the case. The primer has been
assumed to have negligible effects upon these
tests of hole disposition, since the same type
was used for all tests. However, the idea
cannot be discarded entirely that, if other
ignition configurations had been tried, slightly
different results might have been obtained.
The actual data from Table ! 1-2(A) were
adjusted, for ballistic variations, to a condi¬
tion of equal muzzle velocity and recoil
unbalance as indicated in Table 11-2(B). In
adjusting both of these factors, the effective
throat area was changed. These adjusted data
show no large difference in the initial charge
with increasing pressures when comparing
case Type I to III. In making these
adjustments to the data, it was assumed that
the fraction of charge expelled did not change
with charge or pressure level. The adjustment
relations used are
AV m /v m = -o.ibAT e /T e \
*r m /v M = ac 2 /c 2 I
APp/Pp = 1.5 AT e /T e (
A Pp/Pp = (AC 2 /C 2 )[2 + (AC 2 /C 2 )| /
( 11 - 1 )
where
V m = muzzle velocity, fps
T t - effective nozzle thrust, lb
C 2 = charge burnt, lb
P = peak pressure, psi
11-22
AMCP TOti-lW
TABLE 11-2
CARTRIDGE CASE DATA FOR M30A1BD
(A) EvducPon of Perforetion Ditpotiticn in the M30A1B1 Cato
Cam
Chars*.
Hak
Pressure,
Rearward
Recoil,
Velocity.
Propellent Low.
Type
0
P«J
in.
fp*
JB
percent
,
385
6180
6.3
1150
74
19.2
370
7370
9.8
1146
81
21,9
III*
300
9400
35
1245
37
12.3
Ruptured
Prcprilant Lot PA-30191
Ail cant 200 9732-in. hoJ« (00 percent of normal araa)
(B) Evaluation of Data Adjurtad to Condition of Equal Urtbatanoa and Muzzfta Velocity
Cam
Charge,
Raak
Pressure,
Rearward
Rocoil,
Velocity.
ftopellant Lott,
Type
9
P*
iit
<P*
percent
1
385
6180
6.3
1150
74
19.2
II
380
7415
6.3
1150
83
21.9
ill
390
8675
6.3
1150
48
12.3
The information obtained for the 57 mm the breech following firing. Thus, the
rifle cartridge case probably may be extrapo- maximum allowable deformation of the
lated to other rifles within reasonable limits. cartridge case, during the firing cycle, is
However, there an; other factors, such as the determined by the very practical requirement
chamber configuration, which also have to be of removing the spent case through the rifle
considered in the design and modification of a centering ring. This deformation, in turn,
case when a particular weapon is in question. depends on the differentia] pressure which
The ratio of cartridge case perforation are? to develops across the webs of the perforated
nozzle throat area, for most of the cartridge- case and the strength of the case structure,
case-rifle combinations, ranges from 8 to 12. The differential pressure A P is simply
It may not be possible to reduce this ratio by
50 percent, but a 30 percent reduction of &P = P- - P Q (11-2)
perforation area probably can be made-with
no appreciable change in recoil unbalance or where
excessive chamber pressures-and this would
permit a slight reduction in the total charge. P Q - pressure acting on outside of cartridge
case (this may differ from the chamber
pressure ordinarily measured at the
11-17.4 PRESSURE DIFFERENTIAL chamber wall), psi
ACROSS CARTRIDGE CASE
Pj * pressure acting on inside of cartridge
The cartridge case must be removed from case wall, psi
11-23
AMCP7M-2SV
This is illustrated in Fig. 11*13.
Hera again, as in determination of the
ballistic performance of the cartridge case, it
has been found necessary to use experimental
data in order to arrive at definitive results.
Particularly, in the case of differential
pressure determination, the experimental
programs had limited results due to the
difficulty of instrumenting the cartridge case
wall
Experimental results were obtained for the
57 mm Recoilless Rifle, Ml8, with the M30
Cartridge Case. A typical pressure characteris¬
tic is shown in Fig. 11*14. This particular
trace was obtained at the middle position of
the case, although similar results are reported
at both ends of the cartridge case.
Note that two peaks occur in the
differential pressure A/* trace. The first peak,
at about 2 msec before there is any
appreciable chamber (wall) pressure P c , is
probably associated with primer pressure.
This peak, between 400 and 800 psi, falls off
until full ignition, at which time (4*5 msec),
the differential pressure rises to a maximum
just prior to the maximum clumber (wall)
pressure. Both pressures, P c and AP, fall off
rapidly thereafter, A P having attained a
maximum value on the order of 12S0 to 1500
psi and P e a maximum of almost 7000 psi.
In view of the highly empirical nature of
the results presented in this paragraph, it
should be emphasized that they are presented
in order to provide design guidance. At
present, no more adequate model ot these
phenomena is available. Therefore, the car¬
tridge case designer must initiate an experi¬
mental-analytical program geared to his
specific needs.
1M7.fi STRESS ANALYSIS
The stress analysis of the perforated
cartridge case is similar to that used for .many
other configurations, including the tccoitoss
rifle itself. By this, it is meant that the “full”
solution may be built up, starting with the
most basic “membrane solution" and adding
to it the other applicable perturbations-i.e.,
bending effects, dynamic effects, plastic flow,
etc.
In addition to this process of building a
solution from the basic parts, one additional
important factor must be accounted for,
namely, the fact that the cartridge case is
perforated. A simple, direct method that
accounts for the perforations is discussed
later. This is followed by a limited discussion
of some analytical investigations that have
been performed while developing the "full"
perforated cartridge case stress analysis. These
methods are
1. “Equivalent" Pressure and Elastic Con¬
stants for the Perforated Cartridge Case. In an
array of closely spaced perforations, alternate
rows or columns of holes generally are offset
from each other so that each hole may be
imagined to be in the center of a hexagonal
area as shown in Fig 11-15. Thus, the
effective pressure P t acting over the hex¬
agonal area is really only the part of the
differential pressure A P which acts on the
ligaments (the parts of the case left intact
after it is perforated).
Thus, in a method of equivalent clastic
constants, die ligament material is considered
as if it were “spread-out" over the case
periphery; as if the “equivalent" case were
unperforated. Then the equivalent P t is
adjusted to be
P 4 = [1 - (2vr|)/(tf*)lAP, psi (11-8)
where
A P = differential pressure acting on per¬
forated cartridge cases, psi
r p = perforation radius, in.
a, h = distances shown in Fig 11-15, in.
11-24
^T\T7
*% Its
f t t t t
♦ t t
i\ JN i\<\A t\* \£\*\*_\i\_i\ A
net pressure differential UP -_^_\—\p^— y- y, V y U \ 4 J f
expanding gas flow from propellant ignition
within cartridge case
~<L
chamber wall
i P
p ° perforated case
* r i
Forward
Figure 11 13l Gas Flow Through Cartridge Case
t
1
tf
tl
i.
■■
•• > ■ V'. '
^ - - 'Y.; >7? - V
> . - ■. 2 ' .? *v ■
AMCP 706-238
Figure 11-14. Pressure Differentia! Across 57 mm Cartridge Case, M30
The elastic constants of the resulting “equiva¬
lent” thin case are adjusted to obtain
“equivalent” elastic constants. Material useful
in this area may be found in Refs. 1, 2, 3, and
4.
In addition to the use of equivalent elastic
constants, a stress concentration factor is used
sometimes to account for the variation in
stress across the ligament. This variation is
approximately parabolic with minimum stress
occurring at the center and maximum stresses
at the edges. To account for stress concentra¬
tion, the nominal or average stress o avg is
modified by the factor
AMCP 706-238
Figure 11-15. Perforation Artsy (Perforations
i ^ 2tl — (2 r t> /d)] %n
to obtain the maximum stress o max .
°W»X = °W {l +2 ^ “ (2r fi /a)] s/1 }, psi
(11-4)
2. Membrane Solution (Hoop Stress).
Assuming the perforated cartridge case shell
to be represented by an equivalent unpsrfora-
ted shell, the hoop stress o hoop is given by
<W = p <r/b e> psi (11-5)
where
r = cartridge case radius, in.
in Cylindrical Cartridge Case of Radius r)
b e = equivalent unperforated cartridge
case wall thickness, in.
fc «Ml|-(2r*/fl>l <U-«>
Hqs. 11-6 and 11-3 can be combined with Eq.
11-5 to relate the nominal or average hoop
stress Ohoop directly to the differential
pressure A P.
Ohoop = [AP(r/6)][n/c(a — Zr p ) 1
x [1 — (2irp)/(ah)] (11-7)
By use of the general expression in Eq. 11-4,
the maximum hoop stress (ohoop)max iS
given
AMCP 706-238
(<*hoc#)max = l AHr/b)][a/(a - 2 r p )\
x [1 - (2ji rj)/(ah)] (11-8)
x {l +2[1 - (2r > /a)] s/2 }, psi
3. Fixed End Condition. In accounting for
cartridge case restraint at either end of the
cartridge case cylinder, it is frequently useful
to make a further assumption, based on the
apparent mechanisms involved in restraining
cartridge case deformation. This assumption
will frequently take the form of a “fixed” end
condition, i.e., zero deformation and zero
rotation at the end. This assumption is, in
many cases, justified due to the relatively
massive nature of the restraining portions of
the rifle and of the cartridge case itself,
compared with the perforated portion. In
addition, the assumption is “conservative”
resulting in an upper bound on longitudinal
bending stresses.
If a more complex idealization of the end
restraint is found justified by a particular
design, it will be necessary to perform an
interaction analysis between the perforated
cylinder and the geometries idealized at the
end in question. The number of possible
combinations of configurations that may
interact are endless. The reader is referred to
the NASA Report TR-R-103 by Johns and
Orange, Theoretical Elastic Stress Distribu¬
tions Arising from Discontinuities and Edge
Loads in Several Shell-Type Structures (Ref.
15).
For the case of fixed end restraint as shown
in Fig. 11-16, however, it has been shown in
Ref. 1 that the maximum end moment M 0
acting on the perforated portion of the shell is
given by Eq. 1 1-9.
0 6(1 -nKl -v*)’
(in. -ib)-in. -1
(11-9)
Q 0 = 2pM 0 , lb-in. -1 (11-10)
wl.ere
p = (3(1 - !#/(*•> 2 )] 1/4
where P e is found from Eq. 11-3. v* is the
effective Poisson ratio of a perforated plate
and is discussed fully in Ref. 1. The exact
definition of v b also is given in this reference.
4. General Perforated Cartridge Case Anal¬
ysis: The preceding discussions use analyses
that have been performed up to this time, in
connection with stresses in perforated car¬
tridge cases. These investigations largely have
been restricted to “membrane” and “fixed-
end” analysis of elastic members. It is to be
emphasized that other avenues of approach
exist and should be pursued in the course of
cartridge case design. These additional ap¬
proaches include
1. Plastic analysis. To determine extent
of deformation of the assure loaded
cartridge case, in connection with removal of
spent ca"e
<
2. discontinuity analysis. To obtain
accurate bending stress estimates near the
ends of cartridge i ase
3. Dynamic analysis. To account for the
time dependency of cartridge case reaction to
extremely short duration loading.
11-17.6 LINERS FOR THE PERFORATED
CARTRIDGE CASE
In selecting a material to serve as a liner (or
seal) for the recoilless rifle cartridge case, the
following criteria must be considered:
1. It must leave minimum unbumed
residue in the gun chamber and barrel after
firing, which could interfere with chambering
and the end shear Q 0 is given by
I*
AMCP 7C'-138
lf-TTTTTTT TtS!!
^ Cm m_ ^
°oi
'^T
*p
K.
\
~o o
Figure 11-16. Perforated Case Force Diagram-Fixed End Conditions
*
!*►
.
m
> &
of succeeding rounds or induce their prema¬
ture ignition.
2. It must produce no excessive smoke,
acrid fumes, or corrosive products on
combustion.
3. It must have good rough-handling
resistance.
4. It must not adversely affect the ballistic
functioning of the ammunition.
5. It must resist ultraviolet radiation,
weathering and temperature shock, and in
general, show good long-term storage stabili¬
ty.
6. It must serve as an effective water vapor
barrier.
7. It must not allow an electrical spark to
be generated due to friction of propellant in
motion.
Further consideration must be given to the
type of propellant, i.e., single- or double-base,
to be contained by the liner. The liner
material must resist the tendency of nitro¬
glycerin to migrate from double-base propel¬
lants into the liner, resulting in a nitrogly¬
cerin-starved propellant and an overly plasti¬
cized liner.
In the case of an adhesively fastened liner,
the adhesive bonds and joints must resist
rough handling, temperature shock, weather¬
ing, and long-term storage, and be unaffected
when in contact with double-base propellant.
11-17.7 MATERIALS FOR LINERS
Two types of adhesively fastened liner
material have been developed, namely,
1. Polyethylene Terephthalate (PETP)*.
PETP shows good resistance to nitroglycerin
migration and is therefore suitable for use
with double-base as well as single-base
propellants. Suitable thickness range-from
the standpoint of rough-handling resistance,
residue problem, and ballistic pcrformance-is
0.003 to 0.006 in. Cartridge cases must be
free of sharp burrs to prevent puncturing of
the PETP film; and, to prevent electrical
charge build-up, all metal parts of the
complete round should be electrically con¬
nected by removing the anodizing from the
threaded connections. The PETP cartridge
case liner is an adhesively fastened internal
liner of PETP film.
2. Cellulose Nitrate (CN). This material has
poor resistance to nitroglycerin migration,
and cannot be used untreated as a liner for
cartridge cases containing double-base propel¬
lant, although, in principle, a thin coating of
ethyl cellulose applied to the CN seal could
serve as a barrier to nitroglycerin migration.
*MyUr (Type A), E. I. Du Pom, DcNcmours & Co.
11-29
mimas&ssssisaf&ssjtsz.
AMCI* 706-238
The adhesively applied liners find their
greatest application in rounds containing
fixed- and folding-fin stabilized projectiles. In
rounds containing spin-stabilized projectiles,
the propellant is contained in a polyethylene/
rayon laminated bag that is positioned around
the cartridge primer.
The RS type cellulose nitrate is considered
the most suitable of the commercially
available lacquer types. It has the highest
percentage of nitrogen, 11.8 to 12.2, as
against 11.2 to 11.7 for AS type and 10.7 to
11.2 for SS type -ellulose nitrate. It has
higher burning rates, lower water vapor
permeability, higher tensile strength, and
better physical and chemical stability than the
latter types. While high-viscosity CN is
suitable for the production of extruded
tubing, the 5- to 6-sec CN is the preferred
viscosity range for dip solutions. A viscosity
higher than 5-6 sec gives solutions too viscous
at solid content liigh enough for dip
application of the liner. The CN must be
plasticized (e.g., with approximately 10
percent by weight of dioctyl phthalate) to
make the film more pliable and distensible. In
addition, plasticization of CN film serves to
reduce moisture absorption and permeability
of the film, and improve resistance to aging or
embrittlement. An oiganic stabilizer, 1.0-1.5
percent by weight (e.g., diethy’dipheny-
lurea*) must be added to CN to improve its
heat stability. Due to the extreme sensitivity
of CN to ultraviolet radiation, approximately
5 percent of lampblack must be incorporated
to obtain effective light stabilization.
11-17.8 APPLICATION OF LINERS
The factors that follow must be considered
in the application of PETP and CN liners to
recoilless rifle cartridge cases. The dip-applied
CN seal is unsatisfactory because of the
tendency to develop cracks around the
periphery of the cartridge case perforations
during long term storage. A dip-coated,
shrink-fitted extruded liner of CN withstands
"Certain substituted phenols (e.g., p-mcthoxyphenol) arc
known to be more effective heat stabilizers (Ref. 20).
prolonged storage satisfactorily and is resis¬
tant to thermal shock. Dip-coating the
extruded CN liner would appear to reduce the
possibility of damaging the seal when
chambering the round, but this has not been
proven conclusively.
A completely internal liner (flat sheet
cemented to the inner wall of the case) is not
subject to damage by the fin portion of the
fin-stabilized rounds. The cartridge case thus
would act as a protective barrier for CN, and
the possibility of cook-off due to contact of
the liner with hot parts of the gun would be
considerably less than in the case of an
external CN covering. Development of an
internal liner of CN was superseded by the
development of the PETP liner (Ref. 6).
Another advantage offered by an adhesive¬
ly sealed internal liner is a simplified and
more rapid production method (Ref. 7) which
eliminates the solvent hazard associated with
a dip operation. The internal liner also is more
economical. The time required for the
application of the dipped liner is excessive
due to the need for allowing drying periods of
16 hr. In addition, precautions must be taken
to control atmospheric conditions and to
remove dangerous solvent vapors, thus adding
appreciably to the cost and limiting the
ability of certain Army facilities to apply this
type of liner.
Normal procedure in the case of the PETP
liner is for an adhesive to be applied to the
edges of the liner sheet that is precut to size.
The liner is cut from a roll of PETP film that
previously has been shrunk approximately
three percent by heating in an oven at 225° F.
This eliminates shrinkage occurring when heat
is applied later and prevents undue stresses
from developing in the adhesive bond, which
would tend to pull the liner away from the
case wall. After positioning the liner in the
cartridge case, a rubber covered expandable
mandrel is inserted and inflated with air to 1J
psi. The whole assembly is placed in an oven
at 225°F for 15 min to soften the adhesive
and permit it to flow. The case is then
11-30
removed from the oven and allowed to cool,
after which the expander is deflated and
removed.
An external PETP sleeve for recoilless rifle
cartridge cases, applied by using the “shrink”
characteristics of PETP at elevated tempera¬
ture, is not satisfactory (Ref. 8). PETP is not
combustible in the sense that cellulose nitrate
is combustible, and having the metal case
between it and the propellant results in the
sleeves blowing off without burning, leaving
undesirable residue in the weapon after firing.
It should be noted that the CN liner was
developed to replace the polyethylene bag
(Ref. 9) which had low resistance to rough
handling and abrasion (Ref. 5).
11-18 THE FRANGIBLE CARTRIDGE
CASE
11-18.1 GENERAL
The use of a frangible type cartridge case
has many inherent advantages. Some of these
advantages are
1. They weigh considerably less than the
same caliber steel or aluminum case.
2. Their cost is relatively low because of
the use of inexpensive tooling.
3. The simplicity of tooling and know-how
permits their manufacture in practically any
type of shop.
4. Their use eliminates an operation in the
firing sequence, namely, case extraction,
resulting in a higher rate of fire.
5. Finally, they eliminate fired cartridge
case disposal problems.
Though having many advantages in i.s use,
the frangible cartridge case has the disadvan¬
tage of causing gun fouling in certain
applications. In breech-loaded recoilless rifles
■MmMiwn-i-air,— -if )ggEMCSSSiS
AMC? 706-238
where high rates of fire are trying to be
achieved by the use of the frangible case,
residue build-up on parts of the loading
mechanisms has been sufficient to prevent
chambering of the next round. Besides
causing some amount cf residue in any
application, the frangible cartridge case has
the disadvantage of influencing the interior
ballistics of the gun by the manner in which
the burning of the case can affect the
propellant ignition and combustion.
11-18.2 REQUIREMENTS
The design of the frangible propellant
envelope must meet the following minimum
requirements:
1. Maintain chemical, physical, and
dimensional stability over the temperature
range of -65° to +160°F when tested as an
independent item or as a complete round of
ammunition for a period of 300 hr
2. Possess sufficient mechanical strength
to support itself and the propellant charge,
and be capable of resisting rough handling.
3. Produce no corrosive, abrasive, or
toxic effect or exhibit excessive muzzle
smoke or flash when tested in complete
rounds of ammunition.
4. Be chemically compatible with sin¬
gle-base and double-base nitrocellulose and
nitroglycerin propellants and have resistance
toward nitroglycerin migration at tempera¬
tures up to 180°F when tested as an
assembled cartridge.
5. Be resistant to fungus, moisture and
have low vapor transmission to propellant or
other cartridge components.
6. Be chemically compatible with mate¬
rials of the cartridge and packaging container.
7. Shall not ignite under the influence
of electrostatic charge during handling.
yttaMuaMMMHMttllHflimttMbciAiiiukuiu
11-31
AMC? 706-238
8. Will completely disintegrate leaving min¬
imum residue in chamber of the weapon upon
firing.
9. Must not affect weapon functioning
by causing an excessive build-up of pressures,
velocities, and recoils thereby rendering the
weapon unserviceable.
10. Have sufficient strength to withstand
rough handling and resist tearing upon loading
into weapon. It shall be inserted easily
through the breech of the weapon without
any damage occurring which would cause
propellant grains to come into contact with
the weapon breech and/or chamber.
11. Must be constructed of material with
insulating qualities to permit a satisfactory
rate of firing without cook-off occurring.
11-18.3 MATERIALS FOR FRANGIBLE
CASE
Many materials have been tested for use in
frangible cases, but most have had to be
rejected. Kraft paper and cardboard emit
smoldering particles, smoke profusely, and do
not bum entirely. The same problems exist
for polyethylene and Mylar in addition to
their lack of desired or necessary rigidity.
Further, these plastics in general retain the
propellant during the combustion cycle for
too short a period to influence the interior
ballistic functions, and do not act as a heat
sink (as do metal cases) to help in reducing
gun temperature. Additionally, the high
velocity plastic fragments at times damage the
projectile fins.
The one material which appears to possess
most of the desired characteristics is Fiberglas
polyester. Considerable experimental work
was conducted with this material for experi¬
mental 90 mm PAT and 120 mm HAW
weapon systems.
The experimental 90 mm PAT is a
breech-loaded central orifice weapon using a
spring-loaded iris-type nozzle. The nozzle is
spring-loaded so that it enlarges as the
ammunition is chambered, and returns to its
firing position when the ammunition has
passed through the nozzle area. When the
frangible case was used in this weapon,
residue accumulated between the segments of
the iris nozzle rendering it inoperative. Thus,
it was feasible to use the frangible case.
Basically, the same problem existed in the
120 mm HAW weapon s n which the fragments
and residue were excessive enough to prevent
chambering of the following round. Although
these tests were considered encouraging, they
were never carried to completion. It is
believed, however, th..t a successful frangible
type envelope could be developed for future
systems.
11-18.4 THE DAVY CROCKETT CAR¬
TRIDGE CASE
A successful frangible envelope was de¬
signed and employed in the DAVY CROCK¬
ETT weapon system. This, however, should
be considered a special or unique application
and the requirements previously cited do not
necessarily hold in this situation. It is a very
simple application of a frangible envelope.
The DAVY CROCKETT weapon is little
more than a launching tube with an integral
nozzle. In this system, the frangible package
contains the propellant, the ignition system,
and a remote control firing cable. The system
is muzzle-loaded and, therefore, the envelope
is not subjected to any greater shock loading
than those encountered in the standard rough
handling tests. Because of the relatively high
angle of elevation in normal use, any
propellant envelope residue remaining after
firing will drop from the weapon due to
gravity. If any residue shoi/.d remain in the
tube, chambering of the next round will
“wipe" it down into the chamber. Further,
since the rate of fire of this weapon is
contemplated to be relatively low, visual
inspections may be made between each firing.
This application uses phenolic resin, although
11-32
AMCP706238
many other plastics adequately will meet the
requirements.
11-10 THE UNPERFORATED CARTRIDGE
CASE
If a recoilless rifle weapon system were
developed which used an unperforated car¬
tridge case (other than frangible), there would
be several inherent advantages, namely,
1. Manufacture of the cartridge case would
be easier.
2. The cartridge cases would be less costly.
3. Problems with liner materials would be
eliminated.
4. Smaller diameter and less bulky cham¬
ber would be required.
S. Possibly lower strength, lighter weight
cartridge case materials could be used.
One such system developed is the 90 mm,
M67 with Ml 12 Cartridge Case in which the
propellant gases flow directly rearward to the
nozzle. The Ml 12 Cartridge Case is a
relatively simple case. It is drawn from an
aluminum disc and has a straight bore
diameter with a slight external taper to
facilitate extraction. A flange base is provided
for extraction. The case has an anodized
finish to provide erosion resistance to the hot
propellant gases.
A phenolic laminated Kraft paper disc is
employed in the base of the case to position
the projectile in the cartridge case and serves
to provide uniform ignition before rupturing
and discharging through the nozzles of the
weapon. The mouth of the case is crimped to
the projectile to provide a specified bullet
pull.
11-33
AMCP 706-238
SECTION IV
IGNITER
11-20 INTRODUCTION
11-20.1 SCOPE
This section presents methods used n the
design of ignition systems for recoilless rifles.
A general discussion of propellant ignition is
followed by special considerations and design
parameters involved in ignition systems for
recoilless rifles. Ignition system elements and
their functions are discussed, and test and
evaluation procedures outlined.
Emphasis is placed on conventional ignition
systems as used in the majority of current
recoilless rifles. Configurations that have been
developed to satisfy requirements peculiar to
nonconventional or special purpose weapons
or ammunition are presented briefly for
information purposes.
11-20.2 BACKGROUND
Basically, ignition oi propellant is achieved
by application of heat to the surface of the
propellant grains. This application of heat
normally is achieved by envelopment of the
propellant grains in a stream of hot gas. When
the surface temperature of the grain reaches a
certain value T t (ignition temperature) effec¬
tive ignition occurs. The time interval during
which the heat is applied and the rate of heat
application are important factors in attaining
this critical temperature. If the time is too
shore or the rate of application too slow, the
ignition is unstable and effecti'-e burning may
or may not take place. If it does take place, it
does so after a widely variable time delay. The
function of an ignition system is to produce
hot gas with sufficient heat energy and action
time to assure effective and uniform ignition
of tiie propellant charge.
The propellant charge is an anay of
propellant grains with air spaces between and
inside the grains. The hot gas produced by the
ignition system flows through these air spaces
heating the surface of the grains to the
ignition temperature. Ideally, the ignition
system would be so designed as to initiate
combustion simultaneously over the entire
surface of the propellant charge. However, in
practice, the igniter gas does not reach all
surfaces at the same time and may never reach
seme areas at ail. Also, the gas is cooled as it
flows through the propellant charge and may
not retain sufficient heat energy to ignite the
more remote grains. In a closed breech
weapon, if hot gases from the igniter do not
reach all parts of the charge, the combustion
of grains near the igniter produces additional
het gases which aid in the ignition of more
remote parts of the charge. The rise in
pressure accompanying this gas production
further promotes combustion and increases
the effectiveness of total charge ignition. The
recoilless weapon, with its vented chamber
and perforated cartridge case, does not
function in the same manner. If the initial
region of ignition is localized near the igniter
tube, the gas pressure developed tends to eject
the more remote propellant grains from the
case before they have been ignited. A
significant portion of the charge may even be
ejected unburnt or partially burnt through the
nozzle of the weapon. Either of these
occurrences will degrade uniformity of
ballistic performance. Thus, in a recoilless
weapon system, simultaneity of ignition over
as much of the charge as possible is far more
critical than with closed breech systems;
consequently, design of the ignition system
becomes a matter of significance.
General design principles that have been
formulated for rccoilless rifle ignition systems
are presented in the paragraphs that follow.
: i-?5
AM CP 706-238
11-21 IGNITER CONFIGURATION
11-21.1 GENERAL
As shown in Fig. 11-17, a conventional
recoiliess rifle ignitiGn system consists of
three basic elements: primer, secondary
igniter charge, and main igniter charge.
With few exceptions, primers used in
recoilless ammunition are standard small arms
percussion primers.
11-21.2 SECONDARY 1GNJTER CHARGE
The secondary igniter charge (usually
FFFG black powder) acts as a booster charge
for the primer. Due to the relatively large size
of recoiliess rifle igtiiiion tubes, the primer
often does not produce sufficient flame to
assure efficient ignition of the main igniter
charge along its entire length. In this case, the
main igniter charge is initiated at one end and
burning proceeds linearly along the ignition
tube. Since the velocity of a flame front in
black powder is approximately 1300 fps,
burning of this type results in a measurable
time delay between ignition of propellant
grains at the rear of the charge and those at
the front. This results in pressure gradients in
the weapon chamber, ignition delays, and
generally poor uniformity of ballistic perfor¬
mance. The use of a secondary igniter charge
greatly reduces the possibility of this
occurrence by increasing effective primer
output. The combustion of the FFFG, which
is more easily ignited and faster burning than
any of the commonly used main igniter
materials, rapidly produces a high velocity
flame that efficiently ignites the main igniter
charge.
11-21.3 MAIN IGNITER CHARGE
The main igniter charge that produces the
hot gas for propellant ignition, consists
usually of grade A1 black powder. Much work
has been done with other igniter materials
such as zirconium-lead dioxide or harium-
potassium nitrate pellets. Although these
mixtures have a higher caloric output per unit
weight than black powder and are generally
less hygroscopic, they rarely are used in
production systems due to high cost in
comparison with the marginal improvement
over black powder ignition. They do,
however, provide a convenient means of
obtaining effective ignition in cases in which
physical dimensions or strength requirements
of the projectile do not allow sufficient
igniter tube volume to contain the necessary
quantity of A1 black powder.
11-21.4 PRIMER ADAPTER AND IGNI¬
TION TUBE
The remainder of the ignition system
consists of a primer adapter and a perforated
ignition tube usually of aluminum or brass.
Tire prime adapter contains the primer and
secondary igniter charge and. serves to fix the
ignition tube to the cartridge case. In
fin-stabilized ammunition, where the ignition
tube is part of the projectile, the primer
adapter may reive the added function of
providing a shot start for the projectile. The
ignition tube contains the main igniter charge
and is perforated in such a pattern as to
control the distribution of igniter gases to the
propellant charge.
Studies have been conducted and some
limited use made of systems incorporating a
linear source of ignition. These systems
consist of a length of PYROCORE* (small
diameter lead tubing containing a core of
PETN) centrally located and running the
entire length of the main igniter charge with a
primer affixed to the rear end. A cutaway
view of this type of system is shown in Fig.
11-18. Since the velocity of flame propaga¬
tion in PYROCORE is approximately 12,000
fps (10 times that of black powder), this
system most nearly approximates the desired
instantaneous ignition of the main igniter
charge.
Also, since the main igniter charge requires
a lesser degree of confinement to assure
•K. I. DuPont. De Nemours & Company, Inc. Tiudonume
n-36
Percussion
Primer
Secondary Igniter
Main Igniter
Charge —
Perforations
Igniter Tube /
AMCP 709*238
\ & ' j
-i'i j
" 9 >
complete combustion, the ignition tube may
be designed with a greater number of
perforations for better coverage of the
propellant charge. However, in order to
satisfactorily ignite PYROCORE, a detona¬
tion type of primer is required. Primers of this
type usually are electrically or stab actuated
and do not lend themselves to firing by
conventional gun mechanisms. Thus, PYRO¬
CORE ignition systems normally are not used
unless weapon or ammunition design is such
that uniform performance cannot be easily
achieved by more conventional means. For
example, the DAVY CROCKETT system with
its central orifice design and frangible
cartridge case provides an environment in
which the propellant charge had virtually no
confinement. In order to surpo*; combustion
under this condition, euremely rapid initia¬
tion was mandatory. Therefore, PYROCORE
ignition systems were developed and standard¬
ized for this weapon system.
11-21.5 PRIMER
The two types of primers used in recoilless
rifle cartridges are the small arms and artillery
types. The features, advantages, and disadvan¬
tages of both types of primers are detailed in
the paragraphs that follow.
11-21.5.1 Small Arms Percussion Primers
The small arms percussion primer has the
feature of being one of the smallest devices
available for converting mechanical energy
from an appropriate source into chemical
energy in the form of a deflagrating
pyrotechnic reaction. The small arms primer
consists of a metal primer cup into which an
impact sensitive mix is loaded. After covering
the mix with a paper disc, a metal anvil is
pressed into the cup over the mix and paper
disc. Impact by a suitably constructed
hemispherical end firing pin on the primer
cup will locally compress the impact sensitive
mix between the indentation in the primer
cup and the anvil, causing it to deflagrate.
The small size of this component makes it
advantageous in serving as the link between
some form of mechanical firing mechanism
and the ignition of an igniter material such as
black powder. Initiation of combustion by
mechanical means is a key feature of the small
arms primer since other initiating stimuli,
such as electrical energy, frequently require
more complicated initiating mechanisms and
place mote constraints on the weapon system.
Small arms primers are also available with a
variety of chemical formulations depending
upon the type of brisance, corrosive effects,
and temperature storage capabilities required
for a specific application. While the small
arms primer does not produce the large
amount of high-energy gas to ignite effective¬
ly the entire propellant charge in a recoilless
rifle cartridge, it is used effectively as the
means for igniting a material such as black
powder.
If a small arms primer is used in the
initiation sequence, successful detonation of
the primer must occur before any propellant
ignition will take place. Although the small
arms primer has attaineu a very high
operational reliability, there have been cases
in which primers have failed to function
despite the application of an adequate input
energy. The failures have been analyzed, and
in nearly every case were found to be due to
high environmental temperature, excessive
exposure to high environmental temperature,
and/or humidity. During installation of the
small arms primer, the primer anvil is reseated
farther into the primer cup in order to achieve
the proper sensitivity level. Failure to reseat
the primer results in the primer requiring a
considerably increased firing energy fo r
detonation. Reseating the printer anvil to a
point where the priming composition sepa¬
rates until none is present between the cup and
the top of the anvil can result in no
detonation occurring regardless of the amount
of Firing energy applied to the primer.
11-39
AMCP 706-238
11-21.5*2 Artillary-typt Primers
A r tillery-type primers are very similar in
configuration to the igniter configuration
shown in Fig. 11-17, except that no
secondary igniter charge is used. The desired
features or advantages of the artillery-type
primer are very much like those for the igniter
system of par. 11-12, in that the primer tube
can be made as long as the cartridge case
allows in order to radially distribute the
igniter gases throughout the entire propellant
charge.
The problem with using conventional
artillery-type primers in recoilless rifle appli¬
cations occurs in trying to ignite uniformly
the propellant charge in long cartridge cases.
For long artillery primers, the black powder
igniter cnarge within the narrow primer tube
tends to restrict gas flow down the length of
the perforated tube. Without the presence of
a secondary igniter charge to produce a higher
velocity flame front, a time delay exists
between the ignition of black powder closest
to percussion primer and the ignition of black
powder at end of primer tube. This time delay
results in a corresponding uneven ignition of
the propellant charge that surrounds the
primer tube.
Whereaj the conventional closed breech
weapon can sustain a lower pressure rise
during the ignition process and still have an
established ignition of the propellant charge
before the projectile begins to move, the
recoilless rifle must have a rapid rise (3-4
msec) to peak pressure in order to sustain
propellant ignition. At low pressures, the
propellant gases in a recoilless rifle bum
through the cartridge case lin?r and begin to
flow through the case perforations; if ignition
has not occurred throughout the propellant
charge, the resultant pressure loss results in a
slower low-pressure burning of the propellant
which, in some cases, may generate a chamber
pressure that is insufficient to expel the
projectile.
11-22 BASIC DESIGN INFORMATION
In order for the most effective ignition
system to be designed, pertinent characteris¬
tics of the weapon, the projectile, and the
propellant charge must be defined. Most
important to the designer are propellant
composition, burning rate, web size, and
physical dimensions of the propellant pack¬
age. These factors generally are dictated by
system requirements and limitations such as
projectile velocity, acceleration, weapon
strength, and erosion characteristics (see
Chapter S on Interior Ballistics), Thus, the
package selected on this basis may be far from
optimum from a standpoint of effective
ignition due to the following considerations:
1. Propellants of different compositions
vary in ignition temperature and cooling
effect.
2. The burning rate coefficient reflects the
ability of the propellant to support combus¬
tion. Those with higher values support
combustion more readily at low pressures.
3. Propellant grain size and web thickness
control
a. Amount of free space for propaga¬
tion of igniter gases
b. Surface area presented to the igniter
gases
4. Charges naving large diameter with
respect to length increase ignition difficulty.
No quantitative assessment can be made of
the overall effects of these factors on ignition
reliability. Also, technology developed for
one system cannot be applied necessarily to
future developments. Thus, a complete
ignition system development may be required
for any specific recoilless weapon system.
11-40
AMCP 706-238
11-23 DEVELOPMENT PROCEDURE
11-23,1 GENERAL
The discussion that follows is a general
development procedure used successfully for
recoilless weapon ignition systems. Much of
the theory is qualitative in nature and sen es
only as a guide to relative effectiveness of
ignition. Acceptance or rejection of a system
must be based on the only valid measurable
criteria; i.e., uniformity of ignition delay
time, chamber pressure, and muzzle velocity.
It has been determined experimentally that
a black powder chaige of about IOC grains per
pound of propellant is required for effective
ignition. .1 is known that effective ignition
becomes increasingly difficult as igniter
loading density is increased, with ?. value of
0.03 lb/in? considered to be the maximum
allowable. Thus, for a propellant charge of C
pounds the minimum volume of igniter tube
can be calculated. However, the length of the
igniter is limited by the length of the
projectile bcom in the case of a fixed
fin-stabilized round or the length of the
cartridge case for a spin-stabilized round.
Also, for fin-stabilized ammunition, the
igniter tube is part of the projectile in flight,
and restrictions may be placed on the
dimensions of the igniter tube by exterior
ballistic requirements and strength require¬
ments of the projectile. If these considera¬
tions do iot permit an igniter tub'* of the
required volume, conventional ignitci design
procedure cannot be followed. It then
becomes necessary to Investigate the feasibili¬
ty of higher energy igniter materials or more
sophisticated ignition systems. Alternatively,
the weapon system may be re-examined and a
projectile design change effected to permit
increasing igniter tube volume.
11-23.2 DETERMINATION OF HOLE SIZE
AND PATTERN
As mentioned pieviously, it is necessary
that the propellant charge b~ ignited simulta¬
neously over as much of its surface as
possible. Therefore, a«. this point, an igniter
tube hole size and pattern must be deter¬
mined to best achieve this condition. The
equation of state for the gas inside the igniter
tube is taken as
Plv - (C, - N)/p { \ = 12(W - N') F„ psi
(11-11)
where
P = instantaneous space-mean internal
pressure, psi
V - internal volume of igniter tube, in?
C\ = igniter chaige weight, lb
N = weight of igniter gas produced, lb
N 1 = weight of igniter gas discharged from
igniter tube vents, lb
p ( = density of solid igniter material,
lb-in-?
F. = impetus of igniter material,
(ft-lbMlbr 1
The weight /V of igniter gas produced up to
time t is
N t = Pi f <r £ ) i S i dt , lb (11-12)
do
where
(r y ); ~ burning rate (linear) of igniter
material, in .-sec" 1
Si = instantaneous surface area of igniter
material, in?
By use of conventional theory for isen-
trnpic flow of a perfect gas assuming sonic
velocity, the we ; ght N\ of gas discharged
from the igniter tube up to time t is
represented by
AMCF 700-238
(11-13)
The assumption is now made that the
igniter tube pressure remaii>s constant over
the relatively short ignition time t t . Integra¬
tion of Eq. 11-15 over this time results in
where
A p = total area of primer tube perfora¬
tions, in?
ft-ib (11-1G)
7, = ratio of specific heats (c p lc y )j for
igniter gas, dimensionless
g = acceleration due to gravity, ft-sec -2
Simultaneous solution of Eqs. 11-11,
11-12, and 11-13 results in an expression
relating internal igniter tube pressure to total
perforation area for any given weight of
igniter material. However, this method is
extremely cumbersome and, since unknown
igniter gas distribution and heat losses' dictate
'drat final ignition parameters be determined
experimentally, it is unnecessarily precise. By
the use of a few reasonably accurate
assumptions, the perforation area can be
approximated in the manner that follows.
The energy made available to the propel¬
lant by the weight flow dN',/dt of igniter
*.;-es from the igniter tube during the interval
dt can be written (c p ) t T 0 dN 'The igniter
material specific heat at constant pressure
C c p ), is used since the flow process is
isenthalpic. The total energy E A maue
available to the propellant during the time
interval 0 to r is
The ene-gy required E R to raise the surface
of the propellant to its ignition temperature
in time t t is
E r = [c p f>kt t \ in A,(Ti - T a ) (11-17)
where
c = specific heat at constant pressure
propellant, (ft-lbHlb-°Kr‘
p = density of propellant, lb-ini'
k - thermal conductivity of propellant,
(ft-lbHin?-sec-°K/in.T l
A f - surface area of propellant, in?
T, - ignition temperature of propellant,
°K
T a = amoient lemperature, °K
Assume uniform distribution of igniter
gases and negligible heat loss, E A = E R ;
therefore
E A = (cJ
where
,t 0 f m
Jo
/dt)dt
(11-14)
(cJ { T 0 PA p
1/2
U
T a = isochoric flame temperature of pro¬
pellant gas, °K
Substituting from Eq. 11-13
= (c p pkt t ) in A s (T { - T a ) (11-18)
Eq. 11-18 is a general solution for ignition
sysicins. However, in recoiiiess weapon
systems, several of these characteristics vary
slightly over the range of propellants and
igniters used. Thus, the equation can be
greatly simplified by insertion of average
values for these characteristics. For igniter
gases,
11-42
AMCP 706-238
(c p ) t = 620 (ft-lbMlb^KF 1
11-23.3 SAMPLE CALCULATIONS
T = 2500°K
o
7, = 1-25
F ( = 80,400 (fi-lbHlbr*
Likewise, for a typical double-base propellant
The total surface area A, of any propellant
charge can be expressed in terms of physical
dimensions of a single grain, charge, weight,
and density.
A*
4C D + tj£W 11
p D 2 — ttpU) 2 + 2Lj ’
in?
( 11 - 22 )
c p = 540 (ft-lbHlb-°Kr 1
T, = 900° K
T a - 200°K (lowest conditioning tem¬
perature)
p = 0.056 lb-im 3
k = 7.8 X 10- J (in-sec- 0 KJ"' (ft-lb)-
(in. 2 -sec-°K/in.r‘
where
C = propellant weight, lb
p = density of solid propellant, lb-in" 3
D - diameter of propellant grain, in.
w = propellant web, in.
L = length of propellant grain, in.
Thus, for the specific case of recoilless
weapons, Eq. 11-18 reduces to
n p = number of perforations in propellant
grain
PA p t { = 0.0166,4, vT]
or
(11-19)
Perforation areas obtained using Eq. 11-21 are
normally within ±15 percent of the area
actually required.
0. 0166A«
WT, 1
in?
(11-20) Two sample calculations are shown, and
the calculated areas compared with the actual
areas:
It has been determined empirically that in
order to assure uniform ignition, t t should be
2 msec or less and this value normally is used
for igniter calculations. Also, internal igniter
tube pressure should be held to approximate¬
ly 1000 psi. This is nigh enough to assure
uniform burning of the igniter material yet
low enough to preclude any serious structure,
problems in igniter tube design. Substitution
of these values into Eq. 11-20 results in an
expression for perforation area A p as a simple
function of rtrnnp.JIant surface area A _
. r r ' *
Ap = 3.71>< 10" 4 A s , in? (11-21)
1. Example 1: 57 mm Ml8 Recoilless Rifle
C = 0.741b
w = 0.021 in.
D - 0.063 in.
L = 0.271 in.
n p = 1
From Eq. 11-22
= 4 (°- 74 > f 0.063 + 1(0.021) 1 ]
' Aj 0.056 [(0.063)* - 1(0.021) z + 2(0.271)J
= 1356 in?
11-43
AMCP 706-238
From Eq. 11-21
A > =3.7lxl0- 4 (1356) =0.503 in?
The actual perforation area used in the
Ml8 System is 0.60 in? Thus, the calculated
area is approximately 16 percent low.
2. Example 2: 10S mm M27 Recoilless
Rifle
C = 7.5161b
w = 0.038 in.
D = 0.266 in.
L = 0.612 in.
From Eq. 11-22
, 4(7.5) [(0.266) +7(0.038) 1 ]
A * “ 0. 056 L(0.266) z — 7(0T038) 2 * 2(0. 612) J
= 5137 in?
From Eq. 11-21
A p = 3.71 x ltr^m) = 1.906 in?
The actual perforation used in the M27
System is 1.80 in? The calculated value is
approximately 6 percent high.
11-23.4 SELECTION OF HOLE PATTERN
Once the total perforation area has been
determined, the number and pattern of
perforations must be selected. Ideally, a tube
containing a large number of very small
perforations would provide the most uniform
coverage of the propellant charge. However,
this results in thin, low-energy flame jets that
cool rapidly and do not satisfactorily
permeate the propellant charge. Experimental
investigations have shown that perforations
less than 7/32 in. in diameter are impractical
for recoilless ignition systems.
The effect of increasing the diameter of the
holes while maintaining a constant perfora¬
tion area is to reduce the maximum
propel) nt pressure in all firing temperature
ranges. Increasing the hole diameter reduces
the velocity and temperature of the igniter
gases, resulting in nonuniform combustion of
the propellant charge and a lower maximum
pressure. Further increases in the hole
diameter eventually result in such a low
maximum pressure that “poop” shots begin
to occur during low temperature firing?. It has
been established that, for most cases,
perforations should have a diameter less than
3/8 in.
Once the hole size has been established, the
number of perforations is calculated by
dividing the total perforation area by the area
of a single hole. To obtain uniform
distributions of the igniter gases around the
diameter of the charge, perforations are
arranged in six rows 60 deg apart around the
igniter tube diameter with each row contain¬
ing an equal number of equally spaced holes.
Returning to Example 2 of par. 11-23.3 for
the 105 mm system, the calculated total
perforation area is 1.906 in?. Therefore, if a
hole diameter of 7/32 in. is assumed (area =
0.038 in?), the number of perforations in the
igniter tube N p = 1.906/0.038 = 50.2 * 50.
Thus, for this example, an initial design of six
rows of eight or nine 7/32-in. diameter holes
is indicated. The design as finalized contained
six rows of eight 7/3 2-in. diameter perfora¬
tions.
11-23.5 PRELIMINARY BALLISTIC TEST¬
ING
For preliminary ballistic testing, ig$ ter
tubes of the previously selected geometry are
sectioned and provided with a transparent
window for observation of primer function.
The tubes are loaded with the estimated
charge of inert, simulated black powder (100
grains per pound of propellant) and a live
11-44
AMO* 708-23*
primer and secondary igniter (1 grain per 100
grains of black powder). Internal thermo¬
couples are placed at several points along the
length of the tube for measurement of flame
temperature as a function of time. The
primers are then fired and high-speed motion
pictures taken. Analysis of these films in
conjunction with the temperature records
permits selection of the primer-secondary
igniter combination that most efficiently en¬
velopes the main igniter charge in flame. This
study also serves as a basis for refinement of
primer adapter design.
When a primer-secondary igniter combina¬
tion his been chosen, igniter tubes are then
loaded with the estimated charge of A1 black
powder. These tubes are loaded into sectioned
cartridge cases containing inert propellant
grains of the proper size and configuration.
The igniter tubes are fired, and high-speed
motion pictures again are taken and analyzed.
The analysis considers the following charac¬
teristics:
1. Time from primer initiation to first
appearance of flame jets, and variance in time
of appearance of flame Jets along the length
of igniter tube
2. length of propagation of flame jets
through propellant, and variance in this length
3. Variance in time of propagation of
flame jets to most remote propellant grains
4. Lateral flame coverage of propellant
grains located between igniter tube flash holes
5. Duration of flame jets.
3y use of these characteristics, modifications
in igniter charge and igniter tube design are
studied to optimize flame coverage of the
propellant charge.
Once a preliminary propellant-ignition
package has been selected, several complete
rounds are loaded and fired. Firings are
conducted in a test weapon instrumented for
measurement of chamber pressure as a
function of time. These pressure-time records
are as vital a factor in ignition system
development as in prototype weapon de. : gn.
Fig. 11-19 shows pressure-time records from
two identical rounds fired during early phases
of one development program. The curve of
Fig. 11-1&<A) is typical of normal ignition in
a recoilless rifle. Fig. 11-19(B) represents a
round in which adequate ignition was not
achieved. Notice that the time to reach
maximum pressure in Fig. 11-19(A) is much
shorter than in Fig. 11-19(B). In addition, the
unevenness of this portion of the curve in Fig.
11-19(B) indicates localized pressures which,
as mentioned previously, are not conducive to
good ignition. Some ignition delay is to be
expected due to primer delay and time
required for flame propagation. Within
reasonable limits (t t < 2 msec), the absolute
magnitude of delay time is relatively unimpor¬
tant. Most significant is consistency in both
delay time and in the shape of that portion of
the pressure curve within this time. The
inconsistency of these parameters in the
example given required redesign of the
ignition system.
After the initial firings are completed, the
pressure records are analyzed in conjunction
with records of projectile velocity. The
velocity data are also important since
nonuniform round-to-round velocity indicates
incomplete or erratic burning of the propel¬
lant charge which most likely stems from
inadequate and/or nonuniform ignition. Fur¬
ther refinements of the propellant-ignition
package, if required, are made at this point.
11-23.6 FINAL ENGINEERING TESTING
After preliminary tests have been com¬
pleted and a prototype propellant-ignition
system selected, full scale uniformity firings
are initiated. Large samples of ammunition
are conditioned at several temperatures
ranging from -65° to +I60°F and fired from
11-45
Pressure, psi x 10 Pressure, psl x 10
AH97U-2N
(A) P-T Record, Normal Ignition
(B) P-T Record. Poor Ignition
Figure 11-19. P-T Curves for Good and Poor Ignition
11-46
AMC? 70023*
a completely instrumented weapon. Measure¬
ments cf velocity, chamber pressure, and
weapon recoil are subjected to extensive
s tatistical analysis to assure reliability of
future performance. Also, these firings are
monitored for general function, misfires.
hangfires, or other i n di c ati on of ignition
system inconsistency. If any difficulties are
encountered in these firings, they are resolved
by reversion to the preliminary ballistic test
procedure for further refinement of system
characteristics.
IM
AMCP706-2M
SECTION V
THE FUZE
11-24 GENERAL
A fuze is a device for igniting, detonating,
or releasing the charge of a warhead either
upon impact, at a certain predetermined time,
at a specific distance from the target, or under
other desired circumstances. Essentially, the
fuze must initiate detonation at the optimum
time while assuring that detonation does not
occur prematurely. Therefore, the fuze must
insure the safety of the round during normal
handling while taking advantage of the forces
or effects available during and after launch to
activate and prepare the fuze for firing.
Since the function of HEAT, HE, and HEP
projectiles is the same for both rccoilless rifle
and closed breach weapons, the same type of
fuze is used in both type of rounds.
11-26 TYPE OF FUZING
The discussion that follows is a general
survey of the types of fuzing associated with
various types of rounds. A detailed discussion
of lithe principles of operation and the
problems of the design and development of
these fuzes is contained in Ref. 13.
1. High Explosive Antitank (HEAT). Pro¬
jectiles of this type contain a shaped charge of
itigh explosive to effect the penetration of
armor by means of the “Munroe” effect.
Superquick (SQ) fuze action is required with
tt.ese rounds to preserve the proper standoff
distance required for the shaped charge effect.
The period of time from impact until
detonation of bursting charge is on the order
of 100 jisec. Nose type contact fuzing
generally is employed to achieve this action.
One such fuze employs an explosive element
in the nose with a provision for its detonation
products to be “spit back" through a tube to
the base of the shaped charge. Another means
that has been employed to accomplish this
result is to place a piezoelectric transducer in
the nose and use the voltage generated upon
impact to initiate an electric detonator in the
base of the shaped charge.
2. High Explosive Plastic (HEP). Projectiles
of this type use a base-detonating (BD) fuze
with a functioning delay to allow time for the
plastic filler to flatten against the target prior
to initiation.
3. High Explosive (HE), Smoke, and
Target Practice (TP). These rounds are
generally contact fuzed with a point-detonat¬
ing (PD) fuze. Some HE rounds have been
fuzed with settable mechanical time (MT)
fuzes equipped with a point-detonating
element. This combination provides for an
airburst capability if desired.
11-26 SAFE-ARM SEPARATION
An important function of the fuze saflng
system is to ensure that the detonating pin or
arm is locked in position during handling and
gun firing. After firing setback, this safe-arm
subsystem arms (unlocks) the firing pin when
the desired safe separation distance has been
achieved between the gun and projectile.
AMCP70S-23S
SECTION VI
PROPELLANT
11-27 INTRODUCTION
During the firing of a gun, a mass is
accelerated to a desired velocity and given
direction by the expansion of a gas in the
chamber behind the projectile. In order to
accelerate the projectile to velocities of
greater than 1000 fps in the length of the gun
barrel, a source of energy is required, capable
of generating sufficient pressure within less
than 20 msec; the time available to accelerate
a projectile. Alto, the energy source must be
readily manufactured, easy to transport, and
capable of being safely applied. Because the
time cycle involved is quite small, there is not
sufficient time for the completion of slow
processes such as heat transfer. Solid chemical
propellants meet these energy requirements
while also furnishing the gaseous products to
propel the projectile.
A gun can be fundamentally described as a
heat engine. When the propellant charge is
ignited, gases are generated by the burning of
the surface of each propellant grain and a
rapid pressure increase occurs in the gun
chamber. As the projectile begins to move due
to this pressure increase, the chamber volume
increases, resulting in a decrease in the
chamber pressure. However, the burning rate
of the propellant surfaces is high enough so
that the net effect is a rapid increase in
chamber pressure until a maximum is reached
when the projectile is at a relatively short
distance from the start of the gun barrel
rifling. As the projectile travels beyond this
point, the pressure drops until at muzzle exit,
the pressure is, depending upon propellant
uid gun design, 10 to 30 percent of the
maximum pressure.
While pressure aits to drive the projectile
forward, a rearward force, recoil, also acts to
move the gun backward. In recoilless rifles,
these recoil forces are countered by the
rearward discharge of gases through a nozzle
at the breech. Conditions can thus be
controlled so that there can be a of
recoil and projectile momentums. Since about
half of the generated propellant gas is
discharged from the chamber, only the other
half remains to increase the pressure in the
gun which means that the required propellant
weight for a recoilless rifle may exceed that
for a comparable closed breech rifle by a
factor of as much as three to one.
The purpose of propellant design for a
specific gun is to select both the correct
chemical formulation of propellant material
and the correct granulation, i.e., specification
of the individual grain configuration, which
will give rise to the pressure-time history
required to achieve the specified muzzle
velocity while not exceeding the structural
limitations of the weapon. Whereas these
limitations constitute one set of design
problems, consideration also must be given to
cartridge case volume, nozzle erosion, reduc¬
tion of flash and smoke, and ballistic
uniformity. It may not be possible to satisfy
all of these considerations; therefore, a certain
amount of compromise is necessary.
Cartridge are volume may place limita¬
tions on the amount of propellant charge that
can be used so that a propellant with a higher
level of available energy per unit weight or a
propellant configuration with a higher loading
density may have to be used. The rate of
buminc of the propellant must be controlled
so that the rate of gas evolution does not
develop peak pressures exceeding structural
limitations on the rifle. Burnt propellant
should leave little or no residue that could
corrode the rifle bore, create smoke, or
reduce weapon efficiency.
11-51
AMCS> 708-23*
It is also desirable to use a cooler burning
propellant in order to decrease nozzle erosion
and produce combustion gases that are as cool
as possible at the muzzle in order to reduce
flash and thus prevent exposure of the gun
position by night. By the same token, it is
desirable that the mbustion be smokeless to
prevent obsc>’ don of the target and
revealing of th, gun position by day.
To ensure ballistic uniformity, the propel¬
lant grain should be capable of complete and
uniform ignition, and be uniform in size and
shape. Also, the propellant should be capable
of being stored for periods of time up to the
rated useful life without decomposition or
deterioration which would result in nonuni-
form or erratic ballistic behavior.
11-28 HISTORY
In the early stages of recoilless weapon
development, double-base propellants (M2
and M5\ both single- and multiperforated
grains, were used depending on the system
caliber and bore length. As the weapon
development progressed, however, double¬
base propellants were found to be excessively
erosive in the nozzle areas for weapons
intended to be used for repeated fire. This
condition seriously curtailed weapon life.
Improved ignition systems and a change to a
single-base propellant (M10) increased weap¬
on life threefold.
Subsequent to adoption of M10 Propellant,
a program was initiated to develop an
improved propellant with respect to chemical
stability, pressure-temperature relationship,
and loading density for the 106 mm
Recoilless Rifle. M40A1. Three propellants
were developed for study to satisfy this
requirement-T18, T2S, and T28. Results of
this study were:
1. T18 Propellant was not acceptable as a
substitute because of loading density prob¬
lems.
2. Both T2S and T28 Propellants possessed
excellent storage life.
.1. Use of either T2S or T28 Propellants
resulted in SO percent lower velocities and
pressures, depending upon temperature, than
the M10 Propellant when tested in the 106
mm Recoilless Rifle.
4. The T28 Propellant met the require¬
ments of the 106 mm ammunition while
generating a 1,600 pci lower peak pressure
level when using the same charge amount as
used with the M10 Propellant. The T28
Propellant was given the M26 nomenclature
and recommended for standardization for
other recoilless rifles and guns as applicable
(see Ref. 2).
Although T28 Propellant is recommended
for use in recoilless rifles, highly specialized
systems, such as the DAVY CROCKETT,
have used double-base propellants despite
their highly erosive characteristics. These
propellents were employed because of their
superior ignitability and because higher
impetus propellants were required to meet
system requirements. Also, the concept of
employment of nuclear systems did not
anticipate large numbers of rounds being
fired.
11-29 BASIC CHARACTERISTICS
11-29.1 PROPELLANT COMPOSITIONS
On a composition basis, propellants are
divided into the following three groups:
1. Single-base Propellants. Nitrocellulose is
the principal active ingredient of a single-base
propellant. It may contain a stabilizer (usually
possessing plasticizing properties) or any
other n.aterial in a low state of oxidation.
Inhibiting or accelerating materials such as
metals or metallic salts also may be included.
M10 Propellant is an example r -f a single-base
propellant.
;**V
£:
'.'•■•nS
*-T
.; ; s*
fl
*
2. Double-base Propellants. “Double-base"
generally defines propellant compositions
containing nitrocellulose and nitroglycerin. A
better definition of a double-base propellant
is one containing nitrocellulose and a liquid
organic nitrate that gelatinizes the nitrocellu¬
lose. It also may contain additives similar to
the single-base compositions. Nitroglycerin
propellants have not been used extensively in
the United States as standard propellants
because their high combustion temperature
makes them quite corrosive, reducing the
service life of the gun. Furthermore, they may
possibly be in short supply in an emergency.
M2 Propellant is an example of double-base
propellant.
3 Triple-base Propellants. These propel¬
lant* havr three basic active ingredients
nitrocellulose, nitroglycerin, and nitroguani-
dine-in addition to such other additives as
may be necessary. MIS Propellant is ai
example of a triple-base propellant.
11-29.2 IMPETUS
Impetus is a measure of the energy
available in a propellant composition ex¬
pressed in foot-pounds of energy per pound
of propellant. The impetus is proportional to
the number of moles of gas released per
pound of oropcllant and to the flame
temperature of the gas. A high impetus
propellant would require fewer pounds of
charge to achieve a given muzzle energy than
a propellant with a lower impetus.
11-29.3 FLAME TEMPERATURE
Flame temperature or more accurately
isochoric flame temperature is the tempera¬
ture at which the gas is evolved from the solid
propellant when combustion is at constant
volume. While a high impetus is desirable for
efficiency, a high flame temperature is
undesirable because of resulting increased
erosive and muzzle flash characteristics.
AMCP 704-238
11-29.4 WEB THICKNESS
The web thickness of a propellant grain is
the minimum burning thickness or the
minimum thickness of the grain between any
two boundary surfaces.
The relationship between percentage
change in web thickness and percentage
change in peak pressure is a reciprocal
proportion. For example, a 4 percent decrease
in web thickness of the single-perforated M10
Propellant grain used in the 57 mm M18
Recoilless Rifle System resuits in a 10 percent
increase in peak pressure. Decreasing the web
thickness also causes the peak pressure to be
reached in a shorter time and, therefore, at an
earlier point in the projectile travel. Increasing
the web thickness has the opposite effect and
results in a longer rise time to peak pressure.
11-29.5 BURNING RATE
The rate at which the burning surface
recedes along the normal to the propellant
surface is known as the linear burning rate It
is a characteristic of the propellant and. for a
given composition, depends only on the initial
propellant temperature and the chamber
pressure. Empirically, the linear burning rate
is given by Eq. 11-23.
r £ =ni+6i/»J, in.-sec" 1 (11-23)
where
U|.b, B constants dependent upon propel¬
lant and initial temperature
p c = chamber pressure, psi
n = combustion index, dimensionless
For a given propellant shape, a propellant
composition with a higher burning rate results
in a shorter time to consume the propellant
charge; therefore, peak pressure is reached in
11-53
grease:
a shorter rise time and at an earlier point in
projectile travel.
11-294 PROPELLANT SHAPE
The basic grain shapes are
1. Cylindrical, with one or more perfora¬
tions ninning completely through the pain
from end to end
2. Corda or ribbon*
3. Thin, flat grains in a variety of shapes:
diamond, square, hexagonal, circular, circu¬
lar-perforated. etc.
4. Smoothly spherical (such as ball powd¬
er) rolled, rough-spherical.
in general, the combustion of propellant
grains progresses evenly from the surface
where ignition occurs, through subsequent
layers of the explosive as each layer reaches
the ignition temperature. Thus, propellants
burn only on their exposed surfaces. For a
given gun design, optimum ballistic perfor¬
mance is obtained by the correct rate of gas
production and total burning time. The bum
rate and bum time are determined by
selecting a propellant composition with the
required burning rate and then specifying the
web and proper grain geometry. Selection of
the grain geometry has a definite effect on
rifle performance. As discussed in par. 11-31.
a grain shape whose surface area decreases
during firing will attain a maximum gas
pressure earlier than a grain geometry whose
surface area increases burning rate.
In addition to granular propellants, other
propellants wen. investigated for use in
recoilless systems. Some of those investigated
were cord, sheet, ba'\ and plateau. Generally
speaking, except fr sheet propellan’. these
studies were not i ,ied to completion and
did not shew promise in the weapons studied.
Sheet propellant, however, was investigated
extensively to improve temperature coeffi¬
cients of ballistic performance for recoilless
rifle ammunition. The principal configura¬
tions studied were stacked circular discs and
“scroll” assemblies. Scroll configuration study
was abandoned because of its excessive
erosive burning characteristics. The disc
propellant configuration appeared very
promising and was selected for use in
ammunition for the 90 mm shoulder-fired
PAT M67 System. This ammunition pasted ail
phases of the Engineering Tests and most
phases of the Standardization Test; however,
a serious problem developed when this round
was subjected to the arctic tests. The
propellant in the arctic environment became
brittle and caused excessive ballistic perfor¬
mance dispersion, particularly erratic high
pressures. This condition necessitated a switch
to granular-type propellant, and M26 Propel¬
lant was established for use in this system.
11-30 CHEMICAL AND PHYSICAL CHAR¬
ACTERISTICS
The potential thermal energy of a propel¬
lant when fired in a gun is partially converted
from chemical energy into the kinetic energy
of the projectile. The proportion of the total
available energy that can be used by the
projectile is limited by the length of projectile
travel in the barrel, maximum pressure,
expansion ratio, friction, and heat conduction
energy losses. Given a maximum pressure
limit, the ballistician attempts to maximize
the projectile muzzle velocity-while staying
below pressure limit -through the proper
choice of propellant composition, geometry,
and web thickness.
The web and propellant weight combina¬
tion that produces the maximum velocity at a
specified pressure is the optimum charge.
Table 11-3 lists chemical compositions and
combustion characteristics of various propel¬
lants used in recoilless rifle systems. Curves of
burning rate versus pressure can be found in
the Chemical Propulsion Information Agen¬
cy /M2 Solid Propellant Manual.
11-54
TABLE 115
COMPOCTIOM Of tEVEBAL FftOFELLANT*
M10
rut
T26
M2S (T28)
Propellant fptttftariien
m
m
f AfO-ia FA#3426 f MOW OAC-FD-134
T»1
TJB
TM
Nitrocellulose (NC)
77.45
si .95
06
72.CC
73.25
67 56
3000 26.00
30 JK 36 JO
2150 36.45
% Nitrogen
13.15
13,16
13.15
13.16
13.16
13.16
1220 13.15
1250 1356
1250 1356
Nitroglycerin <NG)
19 50
16.00
19.76
20.00
26.00
43.00
36.00
36.00
Barium Nitrate
1.40
1.40
0.76
0.76
0.75
Potassium Nitrate
0.75
0.76
0.70
0.70
0.70
Potassium Sulfate
t.OQ
Otphenyiamine (OPA)
1.00
Ethyl Cantralite
0.60
0.60
8.60
6.00
6.00
2.00
7.96
7JS
Graphite
0.30
0.30
0.10* •
0.30
0.30
050
Carbon Black
0.20*
0.30
050
Ethyl Alcohol (Residual)
1.50
1 20
1.20
120
0.60
0J0
0.60
Water {Retidual)
0.50
0.30
0.30
020
0.00
0.00
0.00
Isochoric Flame Tamp., °K
3319
3245
3000
2938
3071
3081
3674
3100
3136
Forcef, ft-)b. n b x IQ ' 1
360
362
339
346
363
3S6
387
366
368
Unoxtdited Carbon, %
4
3.4
15
22
0
3.0
2J
Combustibles, %
64.5
69.1
66.1
875
380
S8J
68.1
Heat jf Explosion 0, cal/g
1060
1047
938
910
962
562
1222
867
971
Gas Volume n, moles/g
0.03600
0.03036
0.04 J66
0.04219
0.04133
C.04157
0.03788
0.04246
054222
Ratio of Specific Heats 7
1.2238
1.2258
1.2342
1.2421
1.2373
1.2383
15174
15415
15410
laobaric Flame Temp., °K
2712
2647
2431
2365
2482
2488
3018
2496
2526
Co volume, in? per lb
27.76
29.13
28.66
28.77
26.86
28.16
29.04
Specific Gravity
1.67
1.63
1.62
1.82
1.62
1.82
1.62
•Added
”Gl« 2 « Added
tOb*olct*
AMCP 706-238
11-.r PROGRESSIVE AND REGRESSIVE.
BURNING
The different propellant geometries avail¬
able can be divided into two groups according
to the change in the burning surface area as
burning proceeds in the propellant grain. In
solid propellant grains, such as cords or strips,
the burning surface area continually decreases
during combustion. Propellant grains that
have a continually decreasing burning surface
area are termed regressive types of grains or
are said to exhibit regressive burning.
Multipcrforated grains are examples of propel¬
lant grains which exhibit an increase in
burning surface area during combustion and
are termed progressive grjins. Within these
two classifications there are oegrees of
regressiveness or progressivencss. For exam¬
ple, a single-perforated grain is only slightly
regressive, showing an almost constant burn¬
ing surface, compared to a aoherical or cubical
shape which is highly regressive.
Since the amount of gas evolved depends
upon the amount of surface area being
consumed, it is apparent that the choice of
regressive or progressive propellant will have
the effect of determining at what point in
projectile travel peak chamber pressure will
occur. A regressive grain will produce
maximum pressure earlier in tue projectile
travel, and the maximum pressure will be
higher than a progressive grain of the same
composition.
Sometimes it is even of further advantage
to be able to control the shape of the
pressure-travel curve through changing the
type of burning for a specific propellant
geometry by inhibiting the burning of some
of the grain surfaces. As an example, a
single-perforated grain can be coated on the
outside surface to inhibit burning so that only
a progressive burning oecurs from the inside.
REFERENCES
1. Armour Research Foundation Reports Li
Connection with Project No. L037,
Interior Ballistics and Ignition Study on
Expendable and Mon-Expendable Per¬
forated Cartridge Cases. Interim Report
No. 1 and Final Report.
2. G. Horvay, “The Plane Stress Problem of
Perforated Plates”, J. Applied Mechanics.
Sept. 1952.
3. I. Malkin, "Notes on a Theoretical Basis
for Design of Tube Sheets of Triangular
Layout”, Irons. ASME, April 1952.
4. J. W. Dally and A. J. Durelli, “Stresses in
Perforated Panels”, Product Engineering,
March 1952.
5. F. Einberg and A. J. Tuckerman,
Cellulose Nitrate Seal fur the Recoilless
Cartridge Case, Frankford Arsenal Report
R-1134. May 1953.
6. Chemical Studies for Recoilless Ammuni¬
tion Cases, United Shoe Machinery Corp..
Beverly, Mass., Final Report. Phase 1,
Contract DA-I9-020-ORD-I847, Task III
(Project TS4-4018), 25 June 1954.
7. Lining Studies for Recoilless Rifle Shell
Cases, Part II Production Studies, Uni¬
ted Shoe Machinery Corp.. Beverly,
Mass., Contract DA-19-020-ORD-1847,
Task III (Project TS4-4018), Sept. 1955,
8. Development of Cartridge Case Liners for
Recoilless Ammunition. Emhart Manu¬
facturing Company, Hartford. Conn..
Final Report, Contract DA-19-059-ORD-
1093, 13 November 1952 to 15 Septem¬
ber 1966.
11-56
i wfOk‘0
.10.. 437:. fc* Sollott and l : . Einbeig. In ves t jm-
August 1959. . ■'—V.'-
■ Dpgrt A
Handbook, Ammunitlo'ft &erih'; Sc&Vori '
4, Design for Prop ction.
% : \KpS. '.r&tVscM .iV** - i l •n#..*:.. ** ••■’• ••' * ''■
127* Wt. Zfcnk£wi S&me Element M^ih-
U. H fhetieifail dndPractlcdl
Shell-type Structures, N r ASA Report
TR K 103. ■“ v - ~
BIBLIOGRAPHY
G.*'P; Sollott and' li 'Befger, Polyethylene
Terephthalate Seal- yor thP- Reco'tlcss Rifle 7
Cartridge Case, Frankford Arsenal Report
R-1409, October 1957.
J. Vanlloni and F. linberg. Extruded.
Shrink-Fitted Cellulose Nitrate Seal for
i\c‘--t)illess Cartridge Cases, F rank lord Arsenal
Report R-1233, November 1954.
. ‘ I . 1 ’iV ” ' - } ■- • ■ m'V- '{■:*> ;1 . « . 7 ’ r! "
AMCP 706-244,' -EnjpVieerihg' Tbeslgn Idand-
book, Ammunition Series, Section t, AnlVery
Ammunition * General.
Rconlirss Rifle Talmud 1 Informjtion Index,
19*4-1958 Publications Bulletin, Frankford
Arsenal PBS. September 1959 and Supple¬
ment 1.1 < I4S8-|9 (i 2) h>'J.
N. C. Baurmr Development of M26 Pr< pel-
lent for V 7 mu Racilless Rifle M40AI.
Picatinny Aim. al Report No. DR-TR 3-61.
\( nenJature and Definition .y in the Am¬
munition Area. MIL-STD444 6 Februarv
1959,
Deve/ofonent oj DK> non Battalion ‘Xntnank
Weapons and Interior Hatlotie* fnt tin /V-./i".
oj Recoilless Rifles, Summary Report. Vol I
ARF Project No. L034, ORD Project No
TS4-4020, Contract No. DA*M-OJM'Kl)
1157 July 1, 1954.
Major (ieneral Thomas J. Hayes. Elements of
Ordnance . John Wiley & Sons. Inc., 1938.
AMCP 706-106* -107, -108. Engineering
Design Handbook, Elements of Armament
Engineering, forts Onb / Ww. and Three
J. Comer, Theory oj die Interior Ballistics of
Gv is, John Wiley & Sons* Inc ... 1950.
Statisth a! Aids. Interim P.miphict Number -
'H> .' 0 . Maicnel list Piocl ilures lit p
9i) ,’l.U.K I' S -\iih\ It's? .md { viiua! |i it >
< ‘cmuiand.
\NK P 700-150. Engineering Design Hand¬
book. Ballistic Series. Interior Ballistics of
Guns
AD-827 080, Jacob M. Swotinsky, ht-Tube
Burning Rocket for the Advanced. Light
Antitank Weapon, Technical Report ?r>6l
Picatmny Arsenal. January >968: ' /--’N;
AD-819 37JL % Product ImpfOVt nent Test of
* ''trtritige, tiEA r -ES; MSJ44ji1 f JMmm.
AMCPTM-ZM
AD-674 649, Effect of Lotto* Rate end
Winding Sequence on Fatigue and Rupture of
Pressurized Filament-Wound, Glass-Rein-
forced Plastic Cylinders, Technical Report
3693, Pacatinny Arsenal, Auprst 1968.
AD24698, Development of an Improved
Propellant Igniter System for 57 mm Rifle
Ml 8. Picatinny Arsenal, Technical Report No.
1946,30 July 1953.
AD493 265, Examination of Unfired, Separ¬
ate Loading Propelling Charge Assembly for
105 mm Recoilless Gun L G. 41, Piratinny
Arsenal, Technical Report 1439, 23 August
1944.
AD-489 321, Vincent W. Puleo, Conversion of
57 mm, M306A1, Target Practice Cartridges
to HE Cartridges, Picatinny Arsenal, Techni¬
cal Report No. 3438, Aufust 1 966.
AD-18671, Cartridge, Semi-Fixed, HEP-T,
T81E17 for 105 mm Howitzer M2A1 andM4
and Cartridge HEP-T, M326 for 105 mm
RaeoiMatt Rifle M27, Picatinny Arcane!, 1
October 1952.
AD-471 372. Cartridge. HEAT-T, T43fbr 105
mm Recollkss Gun T19, Picatinny Arsenal, 5
May 1950.
AD-471 730. Cartridge, HE. T-42for 105 mm
Recoilless Gun T19.
AD-431 530, William J. Gaston, Malfunction
Investigation of Cartridge, 106 mm HEAT,
M344A1, with Fuze PIED. M509, Picatinny
Arsenal, Technical Report No. 3144, Febru¬
ary 1964.
AD-422 747, Encyclopedia of Explosives and
Related Items, Pfcaiiruty Arsenal.
11-58
CHATTER 12
MOUNTS
SECTION I
INTRODUCTION
12-1 GENERAL
The rccodlfse rifle mount ii that part of the
weapon system which provides a firm base
during firing, and mobility or portability
during twuport. Since recoiUees weapons
exert tittle or no read) force, mounts axe
needed for holding and positioning only.
Therefore, any simple structure of sufficient
stability such as a tripod, or an appropriately
configured saddle to hold it on the gunner's
shoulder, is adequate to support a weapon of
this type.
The first consideration given to tbs design
of a mount is the specific application. Mount
design depends on whether the weapon is to
be fired from a vehicle, ground, shoulder, or a
combination of these. Thu weight distribution
end configuration of the weapon dictates
whether or not the rifle can be shoulder-fired.
For the shoulder-fired rifle, the mount must
provide for firing from either an upright or
prone position and, typically, consists of an
adjustable monopod located under the barrel
and a folding bipod. In firing from the prone
position, the bipod is in the unfolded
position, providing a fixed three point
support with the monopod as shown in Fig.
12-1. In the upright firing position, the bipod
is folded underneath the weapon and serves as
the shoulder rest.
If the rifle is to be fired from a ground
mount only, it can le mounted on a simple
tripod. If the rifle weighs more than 80 lb the
maximum rllowed by Ref. 1 for a 2- to 3-ft
lift off the ground, it will be necessary that
the mount be a separate piece of equipment
which provides quick mounting and removal
of die rifle from the mount. A requirement
for mobility over short distances also may
dictate dm incorporation of a wheel or wheels
in the base kgs of the mounts. Many of the
medium-caliber tecoilkss rifles have a require¬
ment for both ground and vehicular mounting
so that the mount will need to be strong
enough to sustain the accelerations induced
by vehicular travel. In vehicular or towed
mounts, the vertical transportation forces can
be estimated for one of four conditions
involving transportation over level but rough
terrain.
Load factors for a mount on a sprung
chassis art:
1. 3.0 g’s for maximum speeds of less than
30mph
2. 5.0 g's for maximum speeds of 30 mph
or more
Load factors for a mount on an unsprung
chassis are:
1. 5.0 g's for maximum speeds of Ijss than
30 mph
2. 12.0 g's for maximum speeds of 30 mph
or more
12-1
AMCP 706*238
12-2 SPECIFIC EXAMPLES
12-2.1 M79 MOUNT
As shown in Fig, 12-2 (Ref. 1). the 106
mm rifle mount consists of a wheel-barrow-
tripod-type base assembly, an elevating and
tiring assembly> and a traversing assembly.
The M79 Mount provides a stable base for
using the 106 mm M40A1 Rifle with cal .50
Spotting Rifle M8C on the ground and a*' a
means of mounting the rifle on the body of
1/4-ton 4X4 utility trucks (Ref. 2).
The elevating and firing assembly houses
the controls and mechanisms used to support,
elevate, depress, and fire the 106 mm rifle.
The elevating handle is the control for
elevating and depressing the rifle. Fine
elevation adjustments are made by the firing
and vernier elevating shaft knob which is in
the center of the elevating handle and also
serves as the control for firing both the
m^jor-caliber and spotting rifles. The elevating
cradle assembly incorporates a support and
locking yoke for mounting the 106 mm rifle.
Installed in the elevating cradle are a firing
transfer housing and tang which serve to
actuate the firing cable operating levers for
firing both 106 mm and spotting rifles.
The traversing assembly houses Uie controls
and median isms used to traverse the rifles and
suppirts the elevating and firing assembly.
The traversing handle serves as the control for
traversing the rifles. In the center of the
traversing handwheel is the free traverse
shifting shaft knob that is used to disengage
tlic traversing drive from internal gearing to
permit free traversingul the idles.
The elevating and tiaversing base assembly
provides a stable base for ground mounting
the rifles. The adjustable base lclt and right
Figure 12-2. 106 mm Rif/e Mount , M79
u-mni
AMCMtM M
atm ate basically identical, each having a bate
lundte and btaelpcktag dwap foe l^ckkij^c
mount to trage*. The handles
far, lifting the t qpar of the mpunt by two^a
•*A iwwim t^e iaouat in a *N*hMftg«qcpr,
appr ■ op.. a^teel p me mb led in tfcc r ^a*»
front anp.A bape locket low iacorpoigfrd
into thp |«M| front ana*pcvea to lock theham
left and right aims in either 4is* open position
fat jrouai firing, inatalfk^on on trucks, or
"whedbasrow** positions.
12-&2Tli73 MOUNT
The T173 Mount and Tripod T26 k used to
provide a stable base for using the 106 mm
M40A1C Rifle on the pound. As diown in
Fig. 12*3 (Ref. 2), the Rifle Mount T173
consists of an elevating and firing assembly, a
tr a v ersin g assembly, and an adapter assembly.
The elevating and firing assembly, and
traversing assembly are identical to the
respective as s embl ies of the 106 mm Rifle
)|faui$described in par. 12*2.1. The
between the T173 and M79
Mpu$fc *ce in the base assembly.
‘ UJ tihe S tfl73 Mount incorporates an adapter
ssaymfrly (see Fig. 12-3) which consists
of an adapter plate, three aiUpter
litcW i cottar MMnbtVs and a caottan
jupmUy. This adapter assembly fc designed
to mount and lock Rifle Mount T173 to
Tripod T26. The T26 Tripod k a simple
aluminum tri pod comp o rt of two folding
kgs and one fixed leg as does the M79 Mount
Folding kg rei s«* mttons, incorp o rated in
each foldinc L« am to lock and unlock the
folding leg key, permitting the folding legs to
be locked in an open position for operation
and in a folded portion for stowing. Whereas
the M79 Mount with front hne wheel enables
the 106 mm M40A1 Rifle to be wheelbar*
rowed by two or more men for short
»
w
1
, ,0
1
1
1
■]
4
dh tiricee over even terrain, the T173 Mount
requkestfce teswnl of the 106 mm IMQAIC
Rifle tot moving because of the total system
weight of400 lb
12-2J XM124 MOUNT
The XU 124 Mount configuration described
here contetoe t unique Joystick Syatea that
provides rigid joystick for free traverse and
titration, panOdognm m ym ioo of the
rifle, adjustable friction drag, and provirion
for plaoeaaent of gunner adjacent to the
t ra v er s e axes. This mount concept was
deve l oped and ultimately rejected for the 120
nun Heavy Antitank Weapon System,
XM10SF1 (HAW) and is described here for
design and conceptual information.
The XM124 Mount is a variable ratio
ground-vehicular mount and is derignnd for
engaging stationary as well as moving targets,
with the necessary freedom of movement and
aiming accuracy. As shown in Fig. 12-4 (Ref.
3), the XM124 Mount provided for direct
control of the weapon attitude through the
use of a joystick control. The down-range
pointing joystick provided cither e direct
control ratio of 1:1, which is desirable for
moving targets, or a variable ratio between
9:1 end 36:1 for use on distant fixed targets.
The 1:1 ratio is obtained by releasing the
brake sod locking the release draft assembly
in place to form a rigid syriem. The joystick
control would be then centered and fixed
relative to the rifle. When the brake is
released, the rifle could be mowd in free
traverse and elevation.
The mount is designed as drown in Fig.
12-5 so that the joystick control lever is
operated by the left hand, with the gunner on
the left side of the rifle and adjacent to the
trave r se axis. The gunner's right hand is on a
i ■
trigger mechanism which also serves as a
central handle when tire mount is rim> fine
traverse. This position of the gunner ' permits
maximum sweep of the rifle without operator
discomfort
The base legs of tire XM124 Mount are very
similar to those of tire M79 Mount. With tire
wheeled front base kg, tire XM124 is also
capable of being “wheelbarrowed”. Vehicle
mounting of tire XM124 Mount is also similar
to the M79 Mount
12-24 T234 MOUNT
Fig. 12-6 (Ref. 4) shows the integral
shoulder mount and accessory package
designed for the 9C mm T234 Recoilless
Rifle. The original design of this mounting
assembly—containing the monopod, bipod,
face ritield, firing mechanism, ami right
bracket—specified that the assembly be made
from one molded plastic housing. Jjjecause of
strength requirements, tlte^defjjgn of the
asmmbly was modeled ,tp;.$oni^ .partly of
magnesium and partly of Fiberglas or entirely
ofmagnerium.
The entire assembly dipped onto the
muzzle end of th<s rifle end was locked in
position with a snap ring. A glam lamirate
material padded with e beat-resistant filler
end coveted with an abrasion and tear-resis¬
tant flats doth was selected for the face
shield and tire ritoulder pad. This sandwich
construction provided good thermal insula¬
tion to protect the gunner from a hot rifle
tube. As e result of the inability of this
accessory package to meet shock require¬
ments and maintain proper sight alignment,
this design was dropped in favor of a simple
folding bipod mount, which when folded,
served as the shoulder mount on later
configurations of the T234 Recoflkss Rifle.
12-S
figure 12-4 . Mount,
'4
12-7
Figure 12-6. Two-hand Control (Tracking Handle and Trigear HanA! ¥%Me£f Mounted
i
I
Face Shield
Trigger
Handle Grip
Mount
Housing
Grip Screw
Shoulder Monopod
Shield Retaining
Cap
Adjustable Monopod
Release Plunger
Corrugated Fishpaper
Sleeve
Felt Ring
Battery Oontact
Spring
Firinq Pin
Firing Pin Housing
Trigger Handle Grip
Trigger Adapter Insert
Tr igger Handle Pin
Firing Pin Spring
Self Aligning Sent
Cotter Pin
Locking Ring
Trigger Release Latch
Can Follower Pin
Can Follower Roller
Bushing (Insulating)
Brass Eyelet
Trigger Section Showing Utaer Portion
Trigger Return Spring iiLlfoMEoJ
Figun 12-6. Integral Accessory Package tor 90 mm Recoilless Rifle, T234
ACCESSORY MOUNTING EQUIPMENT
12*3 GENERAL
la a complete recotUess rifle system, the
various acceaoty items that must be fastened
or connected to the rifle include such
a c c essori es as ground mount, vehicular
mount, optical sight, spotting rifle, heat
shield, various handles, and firing mechanism.
Among these, the optical sight and the
spotting rifle deserve particular attention in
that they must be precisely positi on ed on the
malm-caliber rifle, and their positions must be
accurately retained throughout field use of
the weapon. This presents a problem as the
expansion and contraction of the rifle
chamber and tube are considerably greater
than those of conventional weapons of
comparable caliber. The absence of recoil
allows flie weight of the recoilless rifle to be
reduced to a level where the material and wall
thickness of the chamber and tube are the
determining factors. When the rifle tube wall
section is made as light as permissible for a
given material, the change in diameter upon
firing occurs quite early and is quite large. It
can be shown that th.‘ strain in a thin-walled
tube is approximately equal to the yield-paint
stress in tension divided by the modulus of
elasticity of tube material It can be seen from
this, that a 5-in. diameter tube will undergo a
diametral expansion of about 0.02S in., 0.028
in., or 0.038 in. when the tube material is
steel, aluminum, or titanium, respectively.
For a tube that expands and contracts to
this extent, it is not advisable to have enlarged
po r tions or projections to act as fastening
pads integrally machined with the tube. Not
only does this make the machining more
difficult, but die strew concentration pattern,
the asymmetric constriction to the motion of
the projectile, and the deflection of the tube
upon firing are highly objectionable. For
example, any increase in thickness on one side
will, by lowering the stress and strain levels at
that side, cause die barrel to deflect or bow
when subjected to internal pressure. Since this
occurs almost instantaneously (in millisec¬
onds), it sets up transverse vibrations in the
barrel and seriously affects accuracy.
Fastening pads to a barrel that undergoes
expansion and contraction has proven to be
fruitless. Welding of mounting pads to a
high-strength alloy steel gun tube is also
inadvisable because the stresses set up and the
changes inflicted upon the grain structure of
the steel would severely damage the tube wall.
Finally, hydrogen brazing rarely lasts for
more than one shot, and the various cements
that have been tried also fail rapidly.
12-4 MOUNTING METHODS
12-4.1 MODERATELY STRESSED WEA¬
PONS
One means that has proven successful for
moderately-stressed weapons is to install a
thin band around the barrel (see mounting,
bands on tne 75 mm M20 Rifle). The band
must, however, have an expansion capability
equal to or greater than (by an adequate
amount of interference fit) the expansion of
the barrel on firing. If steel is to be used as
the material for the expansion device, it
should have a yield-point strength at least
equal to that of the steel barrel, otherwise the
12-9
fattening device will become looee upon
tepeated firings By employing a high-strength
material with a lower modulus of elasticity,
such as titanium or high-strength aluminum, a
fastening band can be installed around the
barrel and will not become loose. Any band,
however, should not be massive enough to
materially constrict the expansion of the
barrel upon which it is placed, since this will
cause stress concentrations in the barrel wall
and dhUirb the projectile travel. A band
thickness of 25 to 50 percent of the barrel
wall thickness and a band width of 0.75 in. to
1.00 in. should be more than adequate to
hold one end of an accessory mounting
bracket. However, no appreciable longitudinal
or circumferential load is to be applied on the
band since this may cause it to be displaced
relative to the barrel due to vibrations of the
barrel. Hence, without keying to the barrel,
the band cannot be relied upon for accurate
positioning.
This method of fastening is not used for
the higher performance weapons with larger
expansion of the barrel.
1242 HIGHLY STRESSED WEAPONS
The HAW 120 mm XM105E1 is an
example of a highly stressed recoilleas rifle.
The barrel is designed to employ the strain
compensation principle (described in Chapter
10) with a 200,000 psi yield-strength material
that permits the barrel wall thickness to be
less than 0.25 in. In order to provide the
necessary mounting surfaces while allowing
for the radial expansion caused by the high
strain in the barrel during firings, the
accessory sleeve as shown in Fig. 12-7 (Ref. 3)
was adopted for use on the HAW weapon
system.
The use of the accessory sleeve allows the
XM90 Spotting Rifle to be located as dose as
possible to the centerline of the major rifle so
as to provide the smallest turning moment
resulting from recoil of the spotting weapon.
The brackets which mount the spotting rifle
fulfill two functions-recoil is transferred to
the main weapon, and a means of biasing is
provided for best trajectory matching. Biasing
adjustment is made through the linear
adjustment mechanism located in die forward
bracket of the accessory sleeve. This mecha¬
nism provides for both azimuth and elevation
correction. The rear bracket of the accessory
sleeve provides the thrust (recoil) support for
the spotting nfle.
With the use of the accessory sleeve on the
HAW weapon system, the optical fire control
components as well as the gunner's eye are
protected from the radial shock at the
moment of firing. The sight bracket is
mounted with bolts on a pad located on the
near side of the accessory sleeve, under the
thrust bearing bracket. The telescope mount
is attached to this sight bracket which is
located as dose as practical to the center of
rotation of the mount to provide the smallest
poarible dislocation during target tracking.
12-6 MOUNTING REQUIREMENTS
12-6.1 GROUND AND VEHICULAR
MOUNTS
The general requirements for a ground and
vehicular mount are that the mount be light
while sufficiently rugged to withstand the
vibration and shocks induced during transport
by vehicle. Light weight of the mount is
required since the weapon system should be
easily mounted for transportability by the
vehicle and conveniently removable from the
vehicle for quick ground emplacement. Figs.
12*2 and 124 show the ground and vehicular
mounts for the Batallion Antitank Weapon
(BAT) and the Joystick System considered
for the HAW weapon systems, selectively.
Vehicle installation of the XM124 Mount for
the HAW weapon is 9hown in Fig. 12-8.
12-10
12-62 TELESCOPE MOUNT
12-62 SPOTTING RIFLE MOUNT
The telescope mount is an independent
unit which is attached to a supporting bracket
of the major-caliber weapon and holds the fire
control telescope. The mount is required to
securely hold and position the telescope for
ease of use by gunner, while incorporating
provisions for making azimuth and elevation
adjustments to ensure proper boresighting and
boresight retention of the telescope with
respect to the weapon. For the case ofthej'
mount holding an elbow telescope, themohht
will be required to have a device for making
adjustments in the cant of the telescope.-;
The Exploded view of the Ml 10 Telescope
Mount for attaching the M103 Telescope to
the 90 mm MAW M67 Rifle, as shown in Fig.
12-9 (Ref. 5), is typical of the mount design
f'v traight-tube telescopes. The MHO Tele-
wpe Mount features a spring-loaded latch in
a gimbal tube which seats the telescope
quickly, and accurately in the mount.
Rotation of either azimuth or elevation
boresight worm screws actuates the respective
wedge gear which then tilts the telescope
support!'-g gimbal tube to the desired
orientation. Fig. 12-10 shows a telescope
mounted to the major-caliber weapon.
As discussed in par. 12-4.2, the mounts for
the spotting rifle must fulfill the requirements
of transferring the spotting rifle recoil to the
major-caliber weapon and providing for
' biasing adjustment, /pother consideration in
mounting thesp^iftingrifle was. locating the
spotting rifle^as. (dose as possible to the
centerline of th’e major iAfle qp as to minimize
the turning moment caused' by the spotting
rifle recoil. ' "
The ek-act location and type of mounting
for the sp< tting rifle will depend on the type
of spotting rifle, used for the specific
application. For ! the larger, ground-fired
recoilless rifles, such as the BAT and HAW
weapons,' the' spotting rifle is mounted on the
top'of-the major-caliber weapon using two
brackets m described in par. 124.2.
Shoulder-fired recoiliess rifles require the use
of the.;tighter and Smaller spotting pistol. Fig.
lef. 6) shows the type of mount used
XM14 Spotting Rifle in the 90 mm
w** kpon system. As seen in Fig. 12-11,
Mirtymd rear brackets provide for both
cessajy transfer bf recoil to the
major-caliber weapon and., the biasing adjust¬
ment mechanism.
12-13
* . it
. ' X*‘i . *h; v*
■ f&b*. 'Spotting Pistol, XM14 Counted an90mmRjfle, M67 ~ '
-- lifoiKt'iflrtmiiff M ^Piiiali^ l iniai^lnMri! nTwmmrf I si rfB ' H T vi iJim i ll^t r liiii nd 11 M li iii i ii ■ >.3 am?
AMCP 70*230
REFERENCES
1. MIL-STD-1472B, Human Engineering De¬
sign Criteria for Military System, Eautp-
ment and Facilities, 31 Dec 1974.
2. TM 9-1000-205-12, Operation and Organi¬
zational Maintenance Q.SOJjol. Spotting
Rifle M8C; 106 mm Rifles M40A1 and
M40A1C; 106 mm Rifle Mounts T173 and
M79; and Tripod T26, Headquarters, De¬
partment of the Army, Washington, DC, 5
March 19S9.
3. Development of 120 mm Recoilless Heavy
Antitank Weapon System (HAW), Final
Report, Technical Memorandum M64,
Frankford Arsenal, Philadelphia, Pa., 1
April 19S9 through 30 June 1962.
3AJ20 mm Rifle System XM105E1, Heavy
Antitank Weapon (HA W), Notes in Devel¬
opment Type Material, Report POL WS-2,
Frankford Arsenal, December 1962.
4. Recoilless Rifle Systems, Ammunition and
Related Items, Status Report No. 1, Vo'.
IV, Report No. R-1316, Frankford Arse¬
nal, Philadelphia, Pa., 1 January through
31 March 1956.
5. Recoilless Rifle Systems, Ammunition and
Related Items, Status Report No. 1, Vol.
VIII, Report No. R-1553A, Frankford
Arsenal, Philadelphia, Pa., 1 January
through 31 March 1960.
6. Recoilless Rifle Systems, Ammunition and
Related Items, Status Report No. 4, Vol.
VI, Report No. R-1499, Frankford Arse¬
nal, Philadelphia, Pa., 1 October through
31 December 1958.
AMCf 700-23S
CHAPTER 13
FIRE CONTROL
13-1 GENERAL
Rscoittow rifle weapon systems use optical
sighting equipment to establish the rate of fire
to the target or, when the spotting rifle is
incorporated as part of the weapon, to direct
the spotting round to the target. AMCP
706-327, Fire Control Systems—General (Ref.
1) contains information relating to optical fire
control components and sights. The spotting
rifle as a fire control adjunct is discussed in
this chapter and the reader is referred to
Ref. 1 for information relating to the optical
components.
Several recoillcss rifle systems use a
subcaliber rifle, called the spotting rifle, as the
method of fire contol. The spotting rifle is
mounted on the major caliber gun so that the
barrel axes of the two rifles are parallel. In
operation, the spotting rifle is fired at the
target and the point of impact of the spotting
projectile observed visually by a flash of tight
and puff of smoke caused from the
detonation of the incendiary or spotter mix
contained in the spotting projectile. The
spotting projectile often provides a visual
trace that enables the observer to follow the
trajectory of the projectile to the target. In
the event the projectile misses the target, the
position of impact becomes a landmark for
correcting the aim. The lay of the rifle
weapon system is changed successively until a
spotting round impacts on the target. At this
time, the firing sequence of the spotting rifle
ends and the major caliber weapon is then
fired.
Matching of the trajectory, at selected
ranges, of the major caliber projectile by the
spotting round is the principle upon which
the spotting rifle functions as a fire control
device. Thus, if the spotting projectile hits the
target, the major caliber projectile also can be
made to hit the target provided: (1) the lay of
the weapon was not altered after t ..4 spotting
round hit the target, (2) the major caliber
weapon is fired before the target moves to
different position, and (3) the trajectories of
the two projectiles follow a known relation -
ship. This relationship is referred to a;
matching and pertains to a coirapondauci:
between projectiles at some point at a give i
range. The basic problem associated with the
matching process in caused by the inherent
difference in the exterior ballistic characteris¬
tics of the major and spotting projectiles.
Matching of the projectile trajectories
requires that the following relation be met
(Ref. 2):
(13-1)
w/s w/s
(M»jor Caliber Projectile) (Spotting Projectile)
where
C D - drag coefficient, dimensionless
W/S = sectional density, lb-i.n7 J
W = weight of projectile, !b
S - projectile cross-sectional area, in?
In practice, it is impossible to meet the
requirement of Eq. 13-1 exactly and,
AMCP 706-238
consequently, a mismatch in trajectory
results. In order to compensate for the
mismatch, a horizontal bias compensating for
wind and differential spinning effects or a
vertical bias adjustment for difference in
projectile drag is introduced between the
spotting and major caliber rifles. The muzzle
velocity of the subcaliber ammunition is also
adjusted (usually increased) in order to
correct for the projectile’s mismatch. Ideally,
a spotting projectile should be designed with
the same ballistic coefficient as that of the
main round In order that the two trajectories
be matched over the desired range. The
maximum practical ballistic coefficient (see
Chapter 4) of the spotting projectile is less
than that of the major caliber ammunition.
This coefficient can be maximized by
improved streamlining, by increasing W/g
through use of higher density materials, and
by minimizing the caliber of the spotting
projectile.
The DAVY CROCKETT Recoilless Rifle
Systenu, as a result of their unique
configuration, preset.'^d the following listed
problems relating to the spotting rifle design.
J The spotting rifle must be positioned
sufficiently far from the bore a~is to clear the
varnetid st the muscle. In these weapons, the
warhead is greater than bore size, see Fig.
1-14 for example.
2. These weapons are nc t direct fire as ir
the case in other recoilless rifles but are fired
at high angles of efevafk n similar to mortars.
3. The XM29 version 1 of the DAVY
CROCKETT is zone fired, i.e., tvo sets of
semifixed ammunition are provid'd, each for
a different range.
The spotting rifle selected for the XM29
Weapon was. the 37 mm XM77E1. Two sets of
ammunition, M415 and M446, were provided
for this spotting rifle; one round to be used in
conjunction with the short range major
caliber ammunition and the other with the
long range. For the XM28 Weapon, a 20 mm
spotting rifle using the M10! Spotter Round
was standardized. A fin-stabilized spotting
projectile was provided in the two 37 mm and
20 mm spotting rounds.
These spotting rifles and associated ammu¬
nition selected produced a net circular'
probable error (CPE) within established
requirements.
13-2 TYPICAL DESIGNS
13-2.1 106 mm RIFLE, M40 WITH CAL SO
SPOTTING RIFLE, M8C
The cal .SO Spotting Rifle, M8C is mounted
above the 106 mm Rifle, M40 with its barrel
axis parallel to that of the barrel of the
recoilless rifle. This spotting rifle is a
gas-operated, semiautomatic,, magazine-fed,
percussion-fired weapon ushig special cal .SO
spotter-tracer ammunition. In the gas oper¬
ated rifle, a port is provided in the side of the
barrel. A portion of the high pressure
propellant gases behind the projectile is
tapped off through the hole and passes
through an orifice into a gas cylinder when
the spotter-tracer projectile has passed the
port. A thrust is generated as a result of these
acting on an operating rod. This thrust is
applied through a mechanism to provide the
energy required for performing the automatic
functions necessary for sustained firing, These
functions include unlocking the IrnR (device
which holds the cartridge in place during
firing and retracts the cartridge case after
firing), retracting the bolt, and operating the
other elements of the gun mechanism.
The gas-operating mechanism of the cal .50
Spotting Rifle, M8C is an impingement type
of mechanism and consists of a simple gas
cylinder, operating rod, and a bolt of
rectangular cross section which is carried by
ar, inertia slide. The inertia slide is a
cylindrical metal block surrounding the bolt
13-2
AMCP70
on the top and aides. The operating rod
impinges directly on the slide which, through
a series of springs, transfen the inertia of the
operating rod to the bolt for performing the
operations of chambering and extracting the
round of ammunition. One feature of die
Spotting Rifle, M8C is the incorporation of a
needk^vahre type gas regulator in the gas
cylinder assembly. This valve can be adjusted
manually to control the operation of the
weapon. Adjustment of the operating power
was provided in order to correct for
differences in effective rigidity of the various
types of DAT weapon system mounts and the
normal variances between the different lots cf
spotting rifle ammunition (Ref. 3).
the cal .50 Spotting Rifle, M8C has the
following characteristics and design data (Ref.
3):
Weight
261b
Length (overall)
49.441 in.
Barrel Length
32.00 in.
Muzzle Velocity
1723 fps.
13-2.2 120 mm RIFLE, XM106 WITH
SPOTTING RIFLE, XM90E1
The Spotting Rifle, XM90E1 for the HAW
weapon system is a 15 mm. (cal .60)
gas-operated rifle similar to the cal .50
Spotting Rifle, M8C for the 106 mm Rifle,
M40. in order to match the trajectory of the
120 mm projectile, ballistic calculations
indicated that the smallest spotting bullet
which could be used was a cal .60 (15 mm).
As a result, it was necessary to develop the
larger Spotting Rifle, XM90E1 instead of
using the. standardized Spotting Rifle, M8C.
The Spotting Rifle, XM90E1 operates in
much the same manner as the Spotting Rifle,
M8C r The major exception to the operational
similarity is in the gas system of the Rifle,
XM90E1, which is designed to close the gas
port after 0.9 in. of operating rod travel and
use gas expansion to complete the stroke
(Ref. 3). In contrast, the Rifle M8C uses the
complete open gas impingement and expan¬
sion method for stroking the operating rod.
Toe Spotting Rifie, XM90E1 also. incorpo¬
rates a charging mechanism that uses the
mechanism advantage of . the charging handle
in conjunction with a roller cocking system to,
bring the peak charging effort to 34 lb
(reduced from a reasonable value of 65 lb).
The Spotting Rifle, XM90E1 has the
following characteristics and design data (Ref.
3):
Weight
37.01b
Length
: 54.6 in.
Barrel Length
32.0 in.
Muzzle Velocity
1800 fps.
13-3 TYPES OF SPOTTER-TRACER
ROUNDS
During World War it, the Germans
developed an experimental cal .30 observing
bullet containing white phosphorus to pro¬
duce smoke and flash. While not widely used
during the War, the round was later tested in
the US and proved to function fairly v;:ll.
The smoke puff was well defined, bu f the
flash was small and of extreme^ <■ T -'t
duration. The design of the inspar. n? ■*.:»-
anism wr* complicated and did not prove ':
be completely reliable on various types of
terrain. In view of this and the ■ potential
hazard of handling white phosphorus; the
bullet was rendered undesirable for further
study.
Studies were then concentrated upon the
use of red phosphorus,, an allotropic form of
phosphorus which can be handled as a dry
powder. Early in these studies, it was found
that the size and duration of Hash was
13-3
AM9NMS
dependent upoo the ml phosphorus content
of the composition.
A cel JO bullet would be useful for only a
very shut range because of die restricted
volume. As a result, effort was concentrated
on the cal JO size which could contain
approximately twice the rod phosphorus
content of a cal JO build. Development of
the cal .50 bullet eventually led to the design
of a Spotter-Tracer Projectile, M48A1, em¬
ployed In the 106 nun Recoilless Rifle
System, M40AJ (BAT). This wss the
sifandcfdiffieri projectile from which the 10
mm and the 15 nun experimental spotter-
tracer rounds were scaled. The current cal .50
spotter-tracer cartridge has been changed to
M48A2 since the BAT system development.
The most investigated areas have been in
the spotter composition and the fuzing
design. These studies are described in pan.
13-5 and 13-6.
134 EVALUATION OF TARGET DISPLAY
From an examination of the many field
variables involved, it is apparent that smoke
and flash provide valuable information to the
gunner. Since smoke provides the gunner Wi-h
desirable supplementary information, the
spotting round should produce visible smoke
on impact.
Flash perception is governed by the
Bunsen-Roscoe Law (Ref. 2) which states that
the product of intensity of a point source of
light and time is a constant for the production
of a given quality oi perception. The flash
duration is a very weak function of intensity
above 1 millilambert, avenging about 0.04
sec. This means that the spot can be made
more detectable by increasing either the
duration or the intensity or both up to about
40 msec. Above 40 msec, duration is no
longer an effective variable, and intensity
alone determines the visual perception.
134 COMPOSITIONS
Spotting compositions (Ref. 2) for small-
caliber ammunition contain red phosphorus as
tiie principal flash and smoke producing fuel
in combination with barium nitrate, an
oxidizing compound. White phosphorus is
used widely in large chemical projectiles and
other smoke producing devices, and is
probably the most efficient mr ferial for
producing dense white smoke. However, the
low melting and ignition temperatures, 44°C,
of white phosphorus create serious hazards in
handling and, wherever possible, red phospho¬
rus is used instead. Red phosphorus, which is
an etiotropic fom of white phosphorus, has a
melting temperature of 590°C and, depending
on purity, an ignition temperature which
varies from 200° to 280°C. Red phosphorus
can be handled and loaded as a dry powder,
bums more slowly than white phosphorus in
air, requires an oxidizer to bum efficiently,
and fives a longer flash duration when used in
a composition. Red phosphorus has a slightly
higher density than white phosphorus and, on
the basis of an equivalent weight in an
explosive composition, produces an equal
quantity of smoke. For example, the 32-grain
charge of spotter composition IM-144 used in
the Spotter-Tracer, M48A1, contains SO
percent red phosphorus (or 16 grains) which
produces a smoke puff approximately equal
to that of 16 grains of white phosphorus
exploded in the air. White phosphorus
projectiles normally require a fuze mechanism
and a charge of high explosive in order to
bunt the projectile and permit the white
phosphorus to react with the air.
Because of the restricted volume in
small-caliber spotting rounds, it is efficient to
use a composition such as the standard
composition IM-144, which bums with
violence under confinement, thus requiring no
separate charge to burst the metal container
upon initiation. However, preparation, han¬
dling, .uid loading of the red phosphorus
134
cocaporitioas must be dons under strict rules
of safety. Because red pho sp horus compos-
tions of the IM-144 type sre sensitive to
impact and friction; zme stearate, aluminum
stearate, or graphite may be added to the
formula for the purpose of reducing friction
d uring load i ng
In the course of development of the
spotting cartridge far the HAW weapon
system, a number of tests were performed
with such pyrophoric metal powders as
msgnerium, titanium, sod zirconium, in plscc
of red phosphorus (Ref. 3). These incendiary
mixes-of which IM-942, IM-943, and IM-982
are prime exampks-resulted in brighter flash
but leas smoke in comparison with the more
generally used composition I El-444. These
mixtures also required the use of a detonator
to insure proper initiation. However, even
with the poor smoke display, it was felt that
because these mixes were less hazardous to
handle, they also should be considered for use
in the HAW weapon system.
13-6 IGNITION
Ignition of a spotting projectile is designed
to occur when the nose of the projectile
strikes an object. The force of impact
operates a mechanism in the projectile which
initiates the explosive train that in turn bunts
the projectile and expels the pyrotechnic
material. The fuzing mechanism must b fast
enough to function before the projectile
buries into soft earth to preclude obscuration
of flash and smoke. At a range of 1,000 yd,
the Spotter, M48A2 penetrates ordinary field
earth about 3 to 6 in. before exploding.
The most common projectile design for
impact initiation employs a thin, relatively
weak metal note that is crushed easily by
impact to ignite a friction-end-impacl-senst*
five composition contained within the nose of
the projectile. For projectiles containing
incendiaries, where impacting against the
earth is not required, the thin metal nose
design is satisfactory and relatively easy to
manufacture.
Tbs p rotkn ttsoctatad with nunf y o t ri og
projectiles is the failure to achieve an
acceptable level of reliable perfosmance
sgaioft the divcnc set of cond itions under
which the ptcgectife must i gnite* Spotting
projectiles must function against a variety of
terrain including earth, sand, snow, and hard
concrete ra macadam road surfac es —ev en by a
grazing impact. On the other hand, the
spotting round must not function premature¬
ly when fired in heavy rain or whan fired
through tall field grass. In order to maintain
this high reliability, a number of different
kinds of fuzing have been investigated. These
fuze designs are air-gap, stab-pin, dynamite-
filled, and designs for operation by projectile
spin decay and electrical ignition.
The amplest of the fuzing concepts is the
air-gap design. As shown in Fig. 13-1 (Ref. 2),
the air-gap Spotting Projectile, M48A1 is
bettered to function in one or more of the
three following ways: (1) crush-up by straight
or graze impact with hard surfaces; (2) by the
open noee scooping up softer earth, sand, bits
of stones, etc., which are forces »§»«»«* the
incendiary composition; and (3) instantan¬
eous high compression of small column of air
trapped in the air-gap hole causing crash-up of
the incendiary composition. Two other
unconfirmed theories of operation of the
air-gap fuzing concept are that the incendiary
composition is detonated by either a shock-
wave created at the point of impact or the
adiabatic heating of the column of air in front
by sudden compression. The purpose of the
aluminum container for the red phosphorus
spotting composition IM-144 is that (1) it acts
as a covjr to prevent functioning of the
incendiary composition IM-163A by drops of
rain and tali grass, and (2) aluminum is
chemically compatible with the red phospho¬
rus whereas copper is not.
13-S
Tracer
Composition
Cup Closure
Incendiary Mixture
IM-144
IM-63A
Incendiary Composition
HU
mMMMm H
Igniter
Composition
Tracer
Container
Figure 13-1. Bullet, Spotter-tracer, Cel .SO, M48A2
AMCPJQtap
A second type of fuze investigated as a
possible alternative to the air-gap type was the
stab-pin fuze. The stab-pin fuze for the cal .SO
experimental Projectile, T14QE12, (shown in
Fig. 13-2, Ref. 2) is the simplest in design of
the mechanically operated types of fuzes. The
operating sequence of the *tab#n fuze
follows: centrifugal force caused Sv;,» /^jsctile
spin opens the firing pin retaining ring; on
impact, the pin is driven into the stab
sensitive primer that ignites the spotting
pyrotechnic charge. During development
testing of the BAT weapon system, it was
found that the stab-pin type fuze did not give
better overall function than the air-gap design
that was adopted and standardized in the
Spotter Projectile, M48A1 (prior to the
introduction of the current M49A2 version).
During the U-BAT Weapon System devel¬
opment program, a number of different fuze
types were tested for the cal .50 Spotter-
Tracer Bullet. The electrically initiated fuze
was baaed on the principle of using some of
the chemical energy of the burning tracer
mixer to charge a barium titanite crystal and
then discharging this energy on an electrically
sensitive primer to ignite the spotting
mixture. In the actual design, the burning
spotter mixture heated the barium titanite
crystal above its Curie temperature (120°C)
and thus charged the crystal. The crystal was
connected to a 6200 pF storage capacitor by
a 0.003 in. air gap. Upon impact of the
spotter projectile, the storage capacitor was
brought in contact with the primer and its
electrical energy discharged to cause primer
detonation. It was found that the condenser
would attain enough energy to denote the
primer after 500 yd of projectile flight.
The use of the principle of spin decay in
the fuzing mechanism was studied extensively
and resulted in several proposed graze and
arming systems. These fuzes incorporated or
led to desirable features such as detonator
safety through the use of an explosive train
interrupter, greater mass concentration to¬
ward the ogive by placing the spotter charge
between the fuze and tracer, and loading of
the spotter charge as a capitulated unit. A
typical spin decay fuze consisted of a metal
block (graze mass) that contained a stab-sensi¬
tive primer. The graze mass initially is held in
place by an antitwist spring. The action of the
centrifugal force causes the antitwist springs
to move radially outward and release the
graze mass upon being subjected to a specified
spin rate. Upon decay or cpin, a drive spring is
able to push the graze mass with the stab
primer right up against the firing pin so that
even a grazing action would cause the firing
pin to stab the primer.
One of the last types of fuzes tested during
the U-BAT Program was the dynamite-filled
fuze. While the display tests were performed
satisfactorily, it was found that the nitroglyc¬
erin in the various materials studied deterio¬
rated under storage conditions. As a result,
this configuration was not used in production
projectiles.
13-7
Fuze, PD, T253
Figure 13-2. Design of Cai .60 Bullet. T140E12
'ifValfrl-'- - La. v *e*Aj*it?i6bJa3ar
.tm -rm
* * *nwrmm rn mis mf
mm
j it iiiimM ■■1119a'p
AMCP7Cf>2SS
REFEREftlCES
1, AMCP 706-327, Engineering Design Hand¬
book, Fin Control System-General.
2. Recoilless Rifle Handbook (Unpublished),
Frankfort Arsenal, Philadelphia, Pi.
3. Development of 120 mm Recolttess
Weapon System. XM89, Memorandum
Report M59-14-5, Frankford Arsenal,
Philadelphia, Pa., Quarterly Progress Re¬
port No. 4, 1 January 1960 through 31
March 1960.
BIBLIOGRAPHY
Symposium on Recent Progress Of RecOiliess
Rifles and Ammunition . .Held at Midwest
Research Institute, 11*13 January 1954,
Sponsored by the Department of Army, 390
PP-
D. Walters, A Spotting Rifle for the 90 mm
Gun Mounted on the T42 Tank, Report
R-1123, Pitman-Dunn Laboratories, Frank¬
ford Arsenal, Philadelphia, Pa., April 1953.
E. D. Crane, B. Werbel and G. Weingarten,
Development of Pyrotechnic Spotting and
Election Charges for Use In Davy Crockett 37
mm Spotting Round, keport TM 1431,
Picatinny Arsenal, Dover, New Jersey, March
1965,24 pp.
13-9
A m, 4 W W ■j- vA- j
>' X'-tf.
INDEX
A
* .. • i ■ . ’ ' ,'
Accessory mounting, 12-9
Accuracy, 4-5
Aerodynamic coefficients, 4-5,4*9,4-H,
4- 17,4-22,4-25
Aerodynamic drag, See; Aerodynamic force,
drag
Aerodynamic force, 4-7
drag, 4-7,4-21
lift,4-7
magnus, 4^7,4-18,4-19
normal, 4-7
Aerodynamic jump, 4-15,4-18
Aerodynamic moment, 4-7
damping, 4-8
magnus, 4-8
roll damping, 4-8
static, 4-7 ’ J
Aircraft tables, See; Sitfcci tables
“All-burnt” condition, 5-59,10-24
Ammunition design, 11-1,104
Approximate methods (interior ballistics),
5- 11
design tables, S-14,5-17 '
graphs, 5-i9 •
piezometric efficiency, 5-12,5-13
similarity, 5-29
thermodynamic efficiency, 5-11,5-13 ■
B
Ballistic efficiency, 5-12, 5-13
Ballistic parameters, 5-9
determination, 2-13
Ballistic parameters, variation analysis,
5-31
flow factor; 1 5-32
propellant regressveness, 5-33
quickness'factor, 5-33
Ballistic
exterior, See: Exterior ballistics
interior, See: Interior ballistics
terminal. See: Terminal Ballistics
Barrel, See: Tube
BAT (Battalion Antitank Weapon), 1-11,
1-12,1-14,1-15,1-26,1-28,1-30,1-38,
142,144,145,146,2-9,6^21,9-39,
104,10-27, IQ-28,12-10,12-13,13-7
Blast '' ■ ••
damage, 643,1-18,548,6-51,9-23'
ntetturement, 8-26
Blowout disc, '2-3 '
Bore area, 2-3
Boresight grooves, 10-26
Bore-size n'ozzle, 6-39
Bore, smooth, 1-13
BoilrrelCt, 1-13,1-14,10-6,11-12,11-13
Breakwire system, 8-7
Breech, 104,10-19 '
actuator, 10-22 1
design, ‘‘blockback” principle, 1-37
self-ejecting, 1-37,144' •'
Breechblock, interrupted thread, 1-9,10-20
C .
Cannon, See: Rifle ''
Cartridge case, 11 J 17,10-5,11-51
combustible, 141,11-1 7 11
frangible, 10-24,11-3,11-17,11-31
perforated, 1-7,1-9, 24, 2-5,9-10,10-23,
11-17,11-28
Case liner, 2-3,11-28
Chamber, 10-23,10-6
pressure, 2-13, 2-14, 10-23
volume, 2-3,10-23
Chamber, tajp^red, 1-9
Coil detector,'.See; Solenoid detector
Conservation of momentutfi, See: Momentum
balance
Constant air temperature (interior ballistic*
solution), 547 . . . '
Convergent-divergent nozzle, See; Nozzle, de
Laval
Cook-off, 10*26
Copper crusher gage, 3-17
Critical -essure, 6-9
ratio, See: Critical pressing
1-1 1
INDEX (Corn'd)
D
Dancer zone, 2-6,2*8,6*51
D«a, 1*4,941
DAVY CROCKET, 1-32,1-33,145,2-9,
9-1S. 104,11-32,11-39,11-52,13-2
de Laval nozzle, See: Nozzle, d* Laval
Definitions, 2-3
Design, 5-11,9-1,945, See oho: the specific
parte; e.g., rifle, ammunition, mounts,
firing mechanism
data, 5-9
examples, 2*17,5-13,5-18,5-29,548,
5-55,5-60,1143
Digital computer (ulterior ballistics solution),
5-57
Directional coefficient, 648
Disadvantages, 9-1S
Droop, 10-28
Ducting (nozzle blast), 6-51
E
Error sources (hitting target), 7-5
Example systems, See: System examples
Exhaust velocity, 6-1 *
Expansion ratio, nozzle, See: Nozzle ex¬
pansion ratio
Exterior ballistics, 4-1
F
Field servicing, 9-39
Fins
fixed, 4-17
folded, 4-18
Fire control, 2-9, 7-7, 7-8,7-11,13-1
See oho: Spotting rifle
Firing mechanism, 10-29
Flash
See: Nozzle flash,
suppression, 1-39, 5-85,649
Flow
rate, S-10,6-12
separation, 6-18
Flow spoilers, 5-85
Force, See: Aerodynamic force
Forcing cone, 10-26
Fragment
patterns, 3*18
size, 3-16
speed, 3-16,3-19
Fragmentation, 3-15
Functional diagram, 2-S, 2-6
Fuze, 144,1149
0
Gas
flow, 2-5, 2-7
leakage, 10-27
pressure, internal, 1-38,5-7,10-23
temperature, 5-7
Gun
expansion ratio, 24
pressure. See: Gas pressure
requirements, 2-13
volume, 2-13
weight, 5-86,10-24,10-25
Gun chamber. See: Chamber
Gun tube, See: Tube
H
HAW (Heavy Antitank Weapon), 1-30,2-17,
10-3. 104,11-3,11-32,12-10,12-13
Heat transfer, 1-38,5-61
Heating, 1-18,10-26
History, 1-3
Hit probability, 2-9,7-5
Human engineering, 9-1,9-37,10-19
I
Igniter, 1-39,11-35
theory, 1141
Indexing, automatic, 11-14
Instrumentation, See: Measurement tech¬
niques
Interior ballistic basic equations, 5-29, 5-35
“all burnt" condition, 5-59
burning, 5-37,542
energy, 541,543
motion, 5-35,542
propellant gas, 5-35,541,543
Interior ballistic equations, heat transfer, 5-61
1-2
■*(
T^"" V'
INOCX (Cant'd)
Interior ballistic equations (coat’d)
solution 5*62
Interior ballistic equations (all burnt)
solution, 5-59
Interior ballistic equations solution, 5-45
constant average temperature, 5*47
numerical integration (digital computer).
5*57
Interior ballistics, 1*39,1-40,5*1,11-19
parameters, 5-8
intermediate flash, 6-49
J
Jet
See: Nozzle jet
See: Warhead jet
Jump (gun), 7-5
See afso: Aerodynamic jump
K
Kill probability. 2*9,7-23
types, 7-23
L
Liner
See: Case Liner
See: Warhead liner
Loading density, 2-4
M
Maintainability, 9-1,9-49
Matching (spotting rifle), 7-9,13-1
Materials, 9-44
MAW (Medium Antita\ < Weapon), 1-20
Measurement techniques, (exterior ballistics)
span, 8-25
yaw, 8-24
Measurement techniques (interior ballistics),
8-1
acceleration, 8-22
premise, 8-17
recoil 3-23
strain, ^21
temperature, 8-24
veloci'y, 8-5
Moby-Dick See: Projectile, T171 .
Momentum
balance, 2*5, 6*5,9-2
conservation, 2-5,6-5,9-2
ratio, 6*2!, 6*22,6*25
Mott equation, 3*15
Mount design, 12*1
Muzzle
energy, 2-11,2-12,2-14
flash. 5-48,5-85
momentum, 2-12
velocity, 7-18, ^*25,8-5,10*23
N
Nozzle, 1-9,1*42,2-4,2-5,5-7,5-41,5-81,
!0-7,10^,10-23
blr st, See: Blast damage
damage, 6-31
design, 6*15,6-23,6-27,6-36
efficiency, 6-6,6-18
entrance area, 2-4,6-27,6-29
erosion, 1-18,1*42,2-4,6-23,6-26,6-31
6-37,6-39,6-41,9*2,10*9
chemical, 6*31
melting, 6-31
resistance, 6-32
exit, 2-4
expansion
angle, 2-4,6-24,6*46
ratio, 2-4,6*13,6-6,69,6-22,6-25
flash, 643,644,649,9-23
jet. 644,
life, 6-26,6-31
surface tenvoerature, 6-35
throat area, 24,6-23,6-25
thrust, 6-13,6-17,6-24,6-25
Nozzle brake, 6-39
Nozzle, types,
annular, 1-9,10-14,10-23
central, 2-13,6-28,10-10,10-23.10-24
de Laval, 69, 6-12,10-9,10-12
kidney-shaped, 10-16, 10-23
multiple, 10-13
supersonic: See: Nozzle de Laval
1NIMEX (Coo'd)
m&Totm
o
Obturation, 1-1?,4-17, IMS
ONTOS, 1-14,1-28
Optimization, See; Parameter optimization
P '
Parameter optimization (interior ballistics),
5-51
length, 5-55
weight, 5-51
PAT (Platoon AntiTank rocoitless rifle), 1-19,
1-20,1-22,1-45,10-28,11-32
Photography, high speed, 8-2 3
Piezoelectric gage, 8-18
Piezomctrio efficiency, 2-4,2-15,5-12,5-13,
645,9-1
Pressure gradient (interior ballistics), 5-83 :
Pressure joint, See: Self-sealing joint
Primary fhsh, 649
Projectile, 10-5
envelope, 11-10
travel, 2-4
types, i 1-6,11-7
caseless, 1-38
Projectiles, specific,
HE, 1-19
HEAT, 1-9,1-11, 1-14, 1-15, .1-17,1-19,
1 44,9-39
M54, 1-37
M63, 1-17
M323, 1-15
M325 WP, 1-15
M326 HEP, 1-15
M344,1-14,1-15
M371 HEAT, 1-20,1-21
KA (Rocket Assisted), 1-20
T125HE, 1-18
Ti 18, 1-13
T! 18210, M3
T119,1-14
T119.E11,1-14,7-10
Tlii'IEAT, 1-14,1-15,7-10
T131, l-i!8,1-20
T138, 1-14
T138E57 HEAT, 1-15
T139 HEP, 1-11,7-10
T139 WP.Ml <
T139E36 HEP, See: M326 HEP
T171 (“Moby Dick”), 1-14,144
T184,1-13
T184HEAT, 1-11,1-15
T188 HEAT, 1-18
T249 HEAT, 1-19
T249E6 HEAT, 1-20
T261W?, 1-15
T263HE, 1-15
T268HE, 1-11 7-10
T273 HEP RA, 1-20
T274 HEAT RA, 1-20
WP, 1-19
Propellant, 140,11-51,11-55
additives, S-85
burning, 5-7,5-83
ejection, 10-23
loss, unbumt, 5-81
requirements, 2-13
Propellant force, See: Propellant impetus
Propellant impetus, 24,11-53
Propellant weight C4«effident, 5-13, 5-14
PYROCORE, 11-39
ft
Radar, doppler, S-l 1
Recoil cancellation, 6*1, 9-1
theory, 6-21
Recoil compensators, 641
Recoil, dimensionless. See: Momentum ratio
Recording equipment, 8-28
Reliability, 9-1,943
Repeating rifle, 1-26
Rifle components design, 10-1
Rifle design, 10-1
Rifles, specific,
ARF, See: T41
EIK, 1-6, 1-35
Ml 8, See; T15
M 20, Sec. 121
M27,See; T19
M28, See: XM63
M29, See: XM64
M40,1-14, 1-15,1-26, 621, 7-8, 7-12,
10-17, 10-23, 13-2
M40A1: See: T170E3
14
ua
amc rma
INDEX ICoot'd)
Rifles, specific, (cant’d)
M67 : See: T219E4
T1SCM18): 1-4,1-7,1-8,1-9,1-11,1-17,
1-18,5-81,6-21,6-27,6-28,6-30,
6- 39,649,7-8,7-11,9-23,10-17,
10*27,10-28,10-29,11-19,11-24
T-16,14,1-9
T17,14,1-9
Tig i-5 l 1-11
TI9(M27), 1-5,1-9,1-11, 1-12,1-14,1-38,
649,7-8,7-11,10-17
T21(M20), 14,1-9,1-10,1-11,641,649,
7- 8,7-11,9-2,10-17,12-9
T21E4,1-9
T41 (ARF), 1-5,628
T62,14,1-17
T62E1,1-17
T66,14,1-13,1-17
T66E2, 5-78
TU8,1-14
T135,1-6,1-15,1-16,621
T135-7,1-5
T136,1-12,1-13, 1-14,1-15,1-26,1-28
T136E1,1-13,1-26
T136E2,1-6,1-13,1-26
T137,1-6,1-14
T149,1-5,1-19,1-15,10-20
T170,1-14,1-15, 1-26,641,10-27
T170E3 (M40A1), 1-6, 1-15,11-52,
12-3
T184,1-5,1-14,1-19,1-20
T189,1-22,1-24,1-25,1-26
T190,14,1-18
T191,1-18
T192,14
T192E4 (M67), 1-5, 1-11,1*20, 1-21,
7-8,7-12, 10-24,1 1-3, 1 1-33,12-13
T219 PAT, 1-20, 1-22
T230E1,1-36
T230E2,1-36
T234,1-5,1-22,1-23,1-30,12-5
T234E, 1-5, 1-22,1-23,1-30,12-5
T237,1-6,1-22. 1-26,1-27,9-10
T246,1-30
XM28, See; XM63
XM29, See; XM64
XM63,1-6,1-32,1-34
XM64,1-32- '-35,13-2
XM89,1-30
XM105,1-31,10-29,13-3
XM105E1,1-6,12-10
Rifling, 10-25,10-28
Rocket motor, 24 .
Rotrting band, pre-engraved, 1-7,1-9,1-18,
1-19,11-1S
Round, See: Projectile
S
Safety, 9-37,10-19,10-31
Sealing, 10-20
Secondary flash, 649
Self-sealing joint, 1-9
Shaped charge, 3-7,3-8,3-9
Shock wave, 643 .
Siacci tables, functions, and method, 4-30
Side loading, 9-10
Sky screen, 8-11
Solenoid detector, 8-10
Solutions, approximate, See: Approximate
Methods >
Spigot configuration, 1-32, l*3o, 11-6
Spin, 3-8,4-5
slow, 4-18
stabilization, See: Stabilization, spin
Spotting rifle, 1-16,2-9,2-16,7-7,7*9,7*11»
12-3,12-10,12-13,13-1,13-2
T43,1-16,1-26,1-28
T46,1-16
T46E1,1-16
T46E2,1-16
See aho: Fire control
Stability, 4-13,4-5
dynamic, 4-15,4-18
gyroscopic, 4-13,4-S6
magnus, 4-16,4-19
projectile, 4-5,4-13
static, 4-13,4-17
Stabilization
fin, 4-17,11-11,11-14, 11-36
spin, 4-13,7-11,10-12,11-11
Standoff, 3-7, 3-9,3-10
Straight-pipe nozzle, See: Bore-size nozzle
1-5
INDEX (Corn'd)
Strain
compensation, 10*25,11*16
gage, 8*18
Super-PAT, See: PAT
Supersonic nozzle, See; Nozzle, de Laval
System
design, 2*1,9*1
effectiveness, 7*1,9*37
examples, 9*23
integration, 2*1
requirements, 2*9,2*10
T
Target
area, 7*25
hard, 7-25
Taylor-Macoll equation, 4*23
Temperature, gup, 1*38,5-61
multiple-shot solution, 5*64
single-shot solution, 5*64
theory vs experiment, 5-67
Terminal ballistics, 3*1,3*3
Test weapon, 2*15
Theoretical analyses, 3-1
Thermodynamic constants, 5*7
Thermodynamic efficiency, 5-11,5*13
Throat area, 2*13,6-9
Thrust, See: Nozzle thrust
Thrust coefficient. See: Nozzle thrust
Trade-off, 2-3,2-9, 2-13,4-17,9-45
Trajectory calculations, 4-5
flat, 4-28
particle, 4*27
Tube, 10*25,10-6, lb-7
U
U*BAT (Ultimate Battalion AntiTank
Weapon), See: BAT
User, 9-37
V
Velocity measurement, See: Measurement
techniques, Velocity
W
Wall thickness, .3* 1S
Warhead
jet, 3-7,3-9,3*10,3-13
liner, 3-9,3-12
Warheads, 3-7,3-15,3-23,3-3, 7-23,11-7,
11-15,11-49
AP, 3-3
HE, 3-4,3-5,3-15,11-7,11-9
HEAT, 3-4,3-7,7-23,11-7,11-11
HEP, 3-4,3-23,7*23
Weapon system
T165, 1-26,1-29
T166,1-26
(AMCFD-TT)
FOR THE COMMANDER:
AMCP 706-238
ROBERT L. KIRWAN
Brigadier General, USA
OFFICIAL: j Chief of Staff
C. J. HAROLD
LTC, GS
Adjutant General
DISTRIBUTION:
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